from __future__ import unicode_literals

import warnings
from functools import partial
from operator import attrgetter

from django import forms
from django.apps import apps
from django.core import checks, exceptions
from django.core.exceptions import FieldDoesNotExist
from django.db import connection, connections, router, transaction
from django.db.backends import utils
from django.db.models import Q, signals
from django.db.models.deletion import CASCADE, SET_DEFAULT, SET_NULL
from django.db.models.fields import (
    BLANK_CHOICE_DASH, AutoField, Field, IntegerField, PositiveIntegerField,
    PositiveSmallIntegerField,
)
from django.db.models.fields.related_lookups import (
    RelatedExact, RelatedGreaterThan, RelatedGreaterThanOrEqual, RelatedIn,
    RelatedLessThan, RelatedLessThanOrEqual,
)
from django.db.models.query import QuerySet
from django.db.models.query_utils import PathInfo
from django.db.models.utils import make_model_tuple
from django.utils import six
from django.utils.deprecation import (
    RemovedInDjango20Warning, RemovedInDjango21Warning,
)
from django.utils.encoding import force_text, smart_text
from django.utils.functional import cached_property, curry
from django.utils.translation import ugettext_lazy as _

RECURSIVE_RELATIONSHIP_CONSTANT = 'self'


def resolve_relation(scope_model, relation):
    """
    Transform relation into a model or fully-qualified model string of the form
    "app_label.ModelName", relative to scope_model.

    The relation argument can be:
      * RECURSIVE_RELATIONSHIP_CONSTANT, i.e. the string "self", in which case
        the model argument will be returned.
      * A bare model name without an app_label, in which case scope_model's
        app_label will be prepended.
      * An "app_label.ModelName" string.
      * A model class, which will be returned unchanged.
    """
    # Check for recursive relations
    if relation == RECURSIVE_RELATIONSHIP_CONSTANT:
        relation = scope_model

    # Look for an "app.Model" relation
    if isinstance(relation, six.string_types):
        if "." not in relation:
            relation = "%s.%s" % (scope_model._meta.app_label, relation)

    return relation


def lazy_related_operation(function, model, *related_models, **kwargs):
    """
    Schedule `function` to be called once `model` and all `related_models`
    have been imported and registered with the app registry. `function` will
    be called with the newly-loaded model classes as its positional arguments,
    plus any optional keyword arguments.

    The `model` argument must be a model class. Each subsequent positional
    argument is another model, or a reference to another model - see
    `resolve_relation()` for the various forms these may take. Any relative
    references will be resolved relative to `model`.

    This is a convenience wrapper for `Apps.lazy_model_operation` - the app
    registry model used is the one found in `model._meta.apps`.
    """
    models = [model] + [resolve_relation(model, rel) for rel in related_models]
    model_keys = (make_model_tuple(m) for m in models)
    apps = model._meta.apps
    return apps.lazy_model_operation(partial(function, **kwargs), *model_keys)


def add_lazy_relation(cls, field, relation, operation):
    warnings.warn(
        "add_lazy_relation() has been superseded by lazy_related_operation() "
        "and related methods on the Apps class.",
        RemovedInDjango21Warning, stacklevel=2)
    # Rearrange args for new Apps.lazy_model_operation
    function = lambda local, related, field: operation(field, related, local)
    lazy_related_operation(function, cls, relation, field=field)


class RelatedField(Field):
    """
    Base class that all relational fields inherit from.
    """

    # Field flags
    one_to_many = False
    one_to_one = False
    many_to_many = False
    many_to_one = False

    @cached_property
    def related_model(self):
        # Can't cache this property until all the models are loaded.
        apps.check_models_ready()
        return self.remote_field.model

    def check(self, **kwargs):
        errors = super(RelatedField, self).check(**kwargs)
        errors.extend(self._check_related_name_is_valid())
        errors.extend(self._check_relation_model_exists())
        errors.extend(self._check_referencing_to_swapped_model())
        errors.extend(self._check_clashes())
        return errors

    def _check_related_name_is_valid(self):
        import re
        import keyword
        related_name = self.remote_field.related_name

        is_valid_id = (related_name and re.match('^[a-zA-Z_][a-zA-Z0-9_]*$', related_name)
                       and not keyword.iskeyword(related_name))
        if related_name and not (is_valid_id or related_name.endswith('+')):
            return [
                checks.Error(
                    "The name '%s' is invalid related_name for field %s.%s" %
                    (self.remote_field.related_name, self.model._meta.object_name,
                     self.name),
                    hint="Related name must be a valid Python identifier or end with a '+'",
                    obj=self,
                    id='fields.E306',
                )
            ]
        return []

    def _check_relation_model_exists(self):
        rel_is_missing = self.remote_field.model not in apps.get_models()
        rel_is_string = isinstance(self.remote_field.model, six.string_types)
        model_name = self.remote_field.model if rel_is_string else self.remote_field.model._meta.object_name
        if rel_is_missing and (rel_is_string or not self.remote_field.model._meta.swapped):
            return [
                checks.Error(
                    ("Field defines a relation with model '%s', which "
                     "is either not installed, or is abstract.") % model_name,
                    hint=None,
                    obj=self,
                    id='fields.E300',
                )
            ]
        return []

    def _check_referencing_to_swapped_model(self):
        if (self.remote_field.model not in apps.get_models() and
                not isinstance(self.remote_field.model, six.string_types) and
                self.remote_field.model._meta.swapped):
            model = "%s.%s" % (
                self.remote_field.model._meta.app_label,
                self.remote_field.model._meta.object_name
            )
            return [
                checks.Error(
                    ("Field defines a relation with the model '%s', "
                     "which has been swapped out.") % model,
                    hint="Update the relation to point at 'settings.%s'." % self.remote_field.model._meta.swappable,
                    obj=self,
                    id='fields.E301',
                )
            ]
        return []

    def _check_clashes(self):
        """
        Check accessor and reverse query name clashes.
        """
        from django.db.models.base import ModelBase

        errors = []
        opts = self.model._meta

        # `f.remote_field.model` may be a string instead of a model. Skip if model name is
        # not resolved.
        if not isinstance(self.remote_field.model, ModelBase):
            return []

        # If the field doesn't install backward relation on the target model (so
        # `is_hidden` returns True), then there are no clashes to check and we
        # can skip these fields.
        if self.remote_field.is_hidden():
            return []

        # Consider that we are checking field `Model.foreign` and the models
        # are:
        #
        #     class Target(models.Model):
        #         model = models.IntegerField()
        #         model_set = models.IntegerField()
        #
        #     class Model(models.Model):
        #         foreign = models.ForeignKey(Target)
        #         m2m = models.ManyToManyField(Target)

        rel_opts = self.remote_field.model._meta
        # rel_opts.object_name == "Target"
        rel_name = self.remote_field.get_accessor_name()  # i. e. "model_set"
        rel_query_name = self.related_query_name()  # i. e. "model"
        field_name = "%s.%s" % (opts.object_name,
            self.name)  # i. e. "Model.field"

        # Check clashes between accessor or reverse query name of `field`
        # and any other field name -- i.e. accessor for Model.foreign is
        # model_set and it clashes with Target.model_set.
        potential_clashes = rel_opts.fields + rel_opts.many_to_many
        for clash_field in potential_clashes:
            clash_name = "%s.%s" % (rel_opts.object_name,
                clash_field.name)  # i. e. "Target.model_set"
            if clash_field.name == rel_name:
                errors.append(
                    checks.Error(
                        "Reverse accessor for '%s' clashes with field name '%s'." % (field_name, clash_name),
                        hint=("Rename field '%s', or add/change a related_name "
                              "argument to the definition for field '%s'.") % (clash_name, field_name),
                        obj=self,
                        id='fields.E302',
                    )
                )

            if clash_field.name == rel_query_name:
                errors.append(
                    checks.Error(
                        "Reverse query name for '%s' clashes with field name '%s'." % (field_name, clash_name),
                        hint=("Rename field '%s', or add/change a related_name "
                              "argument to the definition for field '%s'.") % (clash_name, field_name),
                        obj=self,
                        id='fields.E303',
                    )
                )

        # Check clashes between accessors/reverse query names of `field` and
        # any other field accessor -- i. e. Model.foreign accessor clashes with
        # Model.m2m accessor.
        potential_clashes = (r for r in rel_opts.related_objects if r.field is not self)
        for clash_field in potential_clashes:
            clash_name = "%s.%s" % (  # i. e. "Model.m2m"
                clash_field.related_model._meta.object_name,
                clash_field.field.name)
            if clash_field.get_accessor_name() == rel_name:
                errors.append(
                    checks.Error(
                        "Reverse accessor for '%s' clashes with reverse accessor for '%s'." % (field_name, clash_name),
                        hint=("Add or change a related_name argument "
                              "to the definition for '%s' or '%s'.") % (field_name, clash_name),
                        obj=self,
                        id='fields.E304',
                    )
                )

            if clash_field.get_accessor_name() == rel_query_name:
                errors.append(
                    checks.Error(
                        "Reverse query name for '%s' clashes with reverse query name for '%s'."
                        % (field_name, clash_name),
                        hint=("Add or change a related_name argument "
                              "to the definition for '%s' or '%s'.") % (field_name, clash_name),
                        obj=self,
                        id='fields.E305',
                    )
                )

        return errors

    def db_type(self, connection):
        # By default related field will not have a column as it relates to
        # columns from another table.
        return None

    def contribute_to_class(self, cls, name, virtual_only=False):

        super(RelatedField, self).contribute_to_class(cls, name, virtual_only=virtual_only)

        self.opts = cls._meta

        if not cls._meta.abstract:
            if self.remote_field.related_name:
                related_name = force_text(self.remote_field.related_name) % {
                    'class': cls.__name__.lower(),
                    'app_label': cls._meta.app_label.lower()
                }
                self.remote_field.related_name = related_name

            def resolve_related_class(model, related, field):
                field.remote_field.model = related
                field.do_related_class(related, model)
            lazy_related_operation(resolve_related_class, cls, self.remote_field.model, field=self)

    @property
    def swappable_setting(self):
        """
        Get the setting that this is powered from for swapping, or None
        if it's not swapped in / marked with swappable=False.
        """
        if self.swappable:
            # Work out string form of "to"
            if isinstance(self.remote_field.model, six.string_types):
                to_string = self.remote_field.model
            else:
                to_string = "%s.%s" % (
                    self.remote_field.model._meta.app_label,
                    self.remote_field.model._meta.object_name,
                )
            # See if anything swapped/swappable matches
            for model in apps.get_models(include_swapped=True):
                if model._meta.swapped:
                    if model._meta.swapped == to_string:
                        return model._meta.swappable
                if ("%s.%s" % (model._meta.app_label, model._meta.object_name)) == to_string and model._meta.swappable:
                    return model._meta.swappable
        return None

    def set_attributes_from_rel(self):
        self.name = self.name or (self.remote_field.model._meta.model_name + '_' + self.remote_field.model._meta.pk.name)
        if self.verbose_name is None:
            self.verbose_name = self.remote_field.model._meta.verbose_name
        self.remote_field.set_field_name()

    @property
    def related(self):
        warnings.warn(
            "Usage of field.related has been deprecated. Use field.remote_field instead.",
            RemovedInDjango20Warning, 2)
        return self.remote_field

    def do_related_class(self, other, cls):
        self.set_attributes_from_rel()
        self.contribute_to_related_class(other, self.remote_field)

    def get_limit_choices_to(self):
        """
        Return ``limit_choices_to`` for this model field.

        If it is a callable, it will be invoked and the result will be
        returned.
        """
        if callable(self.remote_field.limit_choices_to):
            return self.remote_field.limit_choices_to()
        return self.remote_field.limit_choices_to

    def formfield(self, **kwargs):
        """
        Pass ``limit_choices_to`` to the field being constructed.

        Only passes it if there is a type that supports related fields.
        This is a similar strategy used to pass the ``queryset`` to the field
        being constructed.
        """
        defaults = {}
        if hasattr(self.remote_field, 'get_related_field'):
            # If this is a callable, do not invoke it here. Just pass
            # it in the defaults for when the form class will later be
            # instantiated.
            limit_choices_to = self.remote_field.limit_choices_to
            defaults.update({
                'limit_choices_to': limit_choices_to,
            })
        defaults.update(kwargs)
        return super(RelatedField, self).formfield(**defaults)

    def related_query_name(self):
        """
        Define the name that can be used to identify this related object in a
        table-spanning query.
        """
        return self.remote_field.related_query_name or self.remote_field.related_name or self.opts.model_name

    @property
    def target_field(self):
        """
        When filtering against this relation, returns the field on the remote
        model against which the filtering should happen.
        """
        target_fields = self.get_path_info()[-1].target_fields
        if len(target_fields) > 1:
            raise exceptions.FieldError(
                "The relation has multiple target fields, but only single target field was asked for")
        return target_fields[0]


class SingleRelatedObjectDescriptor(object):
    """
    Accessor to the related object on the reverse side of a one-to-one
    relation.

    In the example::

        class Restaurant(Model):
            place = OneToOneField(Place, related_name='restaurant')

    ``place.restaurant`` is a ``SingleRelatedObjectDescriptor`` instance.
    """

    def __init__(self, related):
        self.related = related
        self.cache_name = related.get_cache_name()

    @cached_property
    def RelatedObjectDoesNotExist(self):
        # The exception isn't created at initialization time for the sake of
        # consistency with `ReverseSingleRelatedObjectDescriptor`.
        return type(
            str('RelatedObjectDoesNotExist'),
            (self.related.related_model.DoesNotExist, AttributeError),
            {}
        )

    def is_cached(self, instance):
        return hasattr(instance, self.cache_name)

    def get_queryset(self, **hints):
        manager = self.related.related_model._default_manager
        # If the related manager indicates that it should be used for
        # related fields, respect that.
        if not getattr(manager, 'use_for_related_fields', False):
            manager = self.related.related_model._base_manager
        return manager.db_manager(hints=hints).all()

    def get_prefetch_queryset(self, instances, queryset=None):
        if queryset is None:
            queryset = self.get_queryset()
        queryset._add_hints(instance=instances[0])

        rel_obj_attr = attrgetter(self.related.field.attname)
        instance_attr = lambda obj: obj._get_pk_val()
        instances_dict = {instance_attr(inst): inst for inst in instances}
        query = {'%s__in' % self.related.field.name: instances}
        queryset = queryset.filter(**query)

        # Since we're going to assign directly in the cache,
        # we must manage the reverse relation cache manually.
        rel_obj_cache_name = self.related.field.get_cache_name()
        for rel_obj in queryset:
            instance = instances_dict[rel_obj_attr(rel_obj)]
            setattr(rel_obj, rel_obj_cache_name, instance)
        return queryset, rel_obj_attr, instance_attr, True, self.cache_name

    def __get__(self, instance, instance_type=None):
        if instance is None:
            return self
        try:
            rel_obj = getattr(instance, self.cache_name)
        except AttributeError:
            related_pk = instance._get_pk_val()
            if related_pk is None:
                rel_obj = None
            else:
                params = {}
                for lh_field, rh_field in self.related.field.related_fields:
                    params['%s__%s' % (self.related.field.name, rh_field.name)] = getattr(instance, rh_field.attname)
                try:
                    rel_obj = self.get_queryset(instance=instance).get(**params)
                except self.related.related_model.DoesNotExist:
                    rel_obj = None
                else:
                    setattr(rel_obj, self.related.field.get_cache_name(), instance)
            setattr(instance, self.cache_name, rel_obj)
        if rel_obj is None:
            raise self.RelatedObjectDoesNotExist(
                "%s has no %s." % (
                    instance.__class__.__name__,
                    self.related.get_accessor_name()
                )
            )
        else:
            return rel_obj

    def __set__(self, instance, value):
        # The similarity of the code below to the code in
        # ReverseSingleRelatedObjectDescriptor is annoying, but there's a bunch
        # of small differences that would make a common base class convoluted.

        # If null=True, we can assign null here, but otherwise the value needs
        # to be an instance of the related class.
        if value is None and self.related.field.null is False:
            raise ValueError(
                'Cannot assign None: "%s.%s" does not allow null values.' % (
                    instance._meta.object_name,
                    self.related.get_accessor_name(),
                )
            )
        elif value is not None and not isinstance(value, self.related.related_model):
            raise ValueError(
                'Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (
                    value,
                    instance._meta.object_name,
                    self.related.get_accessor_name(),
                    self.related.related_model._meta.object_name,
                )
            )
        elif value is not None:
            if instance._state.db is None:
                instance._state.db = router.db_for_write(instance.__class__, instance=value)
            elif value._state.db is None:
                value._state.db = router.db_for_write(value.__class__, instance=instance)
            elif value._state.db is not None and instance._state.db is not None:
                if not router.allow_relation(value, instance):
                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)

        related_pk = tuple(getattr(instance, field.attname) for field in self.related.field.foreign_related_fields)
        if not self.related.field.allow_unsaved_instance_assignment and None in related_pk:
            raise ValueError(
                'Cannot assign "%r": "%s" instance isn\'t saved in the database.' %
                (value, instance._meta.object_name)
            )

        # Set the value of the related field to the value of the related object's related field
        for index, field in enumerate(self.related.field.local_related_fields):
            setattr(value, field.attname, related_pk[index])

        # Since we already know what the related object is, seed the related
        # object caches now, too. This avoids another db hit if you get the
        # object you just set.
        setattr(instance, self.cache_name, value)
        setattr(value, self.related.field.get_cache_name(), instance)


class ReverseSingleRelatedObjectDescriptor(object):
    """
    Accessor to the related object on the forward side of a many-to-one or
    one-to-one relation.

    In the example::

        class Choice(Model):
            poll = ForeignKey(Place, related_name='choices')

    `choice.poll` is a ReverseSingleRelatedObjectDescriptor instance.
    """

    def __init__(self, field_with_rel):
        self.field = field_with_rel
        self.cache_name = self.field.get_cache_name()

    @cached_property
    def RelatedObjectDoesNotExist(self):
        # The exception can't be created at initialization time since the
        # related model might not be resolved yet; `rel.model` might still be
        # a string model reference.
        return type(
            str('RelatedObjectDoesNotExist'),
            (self.field.remote_field.model.DoesNotExist, AttributeError),
            {}
        )

    def is_cached(self, instance):
        return hasattr(instance, self.cache_name)

    def get_queryset(self, **hints):
        manager = self.field.remote_field.model._default_manager
        # If the related manager indicates that it should be used for
        # related fields, respect that.
        if not getattr(manager, 'use_for_related_fields', False):
            manager = self.field.remote_field.model._base_manager
        return manager.db_manager(hints=hints).all()

    def get_prefetch_queryset(self, instances, queryset=None):
        if queryset is None:
            queryset = self.get_queryset()
        queryset._add_hints(instance=instances[0])

        rel_obj_attr = self.field.get_foreign_related_value
        instance_attr = self.field.get_local_related_value
        instances_dict = {instance_attr(inst): inst for inst in instances}
        related_field = self.field.foreign_related_fields[0]

        # FIXME: This will need to be revisited when we introduce support for
        # composite fields. In the meantime we take this practical approach to
        # solve a regression on 1.6 when the reverse manager in hidden
        # (related_name ends with a '+'). Refs #21410.
        # The check for len(...) == 1 is a special case that allows the query
        # to be join-less and smaller. Refs #21760.
        if self.field.remote_field.is_hidden() or len(self.field.foreign_related_fields) == 1:
            query = {'%s__in' % related_field.name: set(instance_attr(inst)[0] for inst in instances)}
        else:
            query = {'%s__in' % self.field.related_query_name(): instances}
        queryset = queryset.filter(**query)

        # Since we're going to assign directly in the cache,
        # we must manage the reverse relation cache manually.
        if not self.field.remote_field.multiple:
            rel_obj_cache_name = self.field.remote_field.get_cache_name()
            for rel_obj in queryset:
                instance = instances_dict[rel_obj_attr(rel_obj)]
                setattr(rel_obj, rel_obj_cache_name, instance)
        return queryset, rel_obj_attr, instance_attr, True, self.cache_name

    def __get__(self, instance, instance_type=None):
        if instance is None:
            return self
        try:
            rel_obj = getattr(instance, self.cache_name)
        except AttributeError:
            val = self.field.get_local_related_value(instance)
            if None in val:
                rel_obj = None
            else:
                params = {
                    rh_field.attname: getattr(instance, lh_field.attname)
                    for lh_field, rh_field in self.field.related_fields}
                qs = self.get_queryset(instance=instance)
                extra_filter = self.field.get_extra_descriptor_filter(instance)
                if isinstance(extra_filter, dict):
                    params.update(extra_filter)
                    qs = qs.filter(**params)
                else:
                    qs = qs.filter(extra_filter, **params)
                # Assuming the database enforces foreign keys, this won't fail.
                rel_obj = qs.get()
                if not self.field.remote_field.multiple:
                    setattr(rel_obj, self.field.remote_field.get_cache_name(), instance)
            setattr(instance, self.cache_name, rel_obj)
        if rel_obj is None and not self.field.null:
            raise self.RelatedObjectDoesNotExist(
                "%s has no %s." % (self.field.model.__name__, self.field.name)
            )
        else:
            return rel_obj

    def __set__(self, instance, value):
        # If null=True, we can assign null here, but otherwise the value needs
        # to be an instance of the related class.
        if value is None and self.field.null is False:
            raise ValueError(
                'Cannot assign None: "%s.%s" does not allow null values.' %
                (instance._meta.object_name, self.field.name)
            )
        elif value is not None and not isinstance(value, self.field.remote_field.model):
            raise ValueError(
                'Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (
                    value,
                    instance._meta.object_name,
                    self.field.name,
                    self.field.remote_field.model._meta.object_name,
                )
            )
        elif value is not None:
            if instance._state.db is None:
                instance._state.db = router.db_for_write(instance.__class__, instance=value)
            elif value._state.db is None:
                value._state.db = router.db_for_write(value.__class__, instance=instance)
            elif value._state.db is not None and instance._state.db is not None:
                if not router.allow_relation(value, instance):
                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)

        # If we're setting the value of a OneToOneField to None, we need to clear
        # out the cache on any old related object. Otherwise, deleting the
        # previously-related object will also cause this object to be deleted,
        # which is wrong.
        if value is None:
            # Look up the previously-related object, which may still be available
            # since we've not yet cleared out the related field.
            # Use the cache directly, instead of the accessor; if we haven't
            # populated the cache, then we don't care - we're only accessing
            # the object to invalidate the accessor cache, so there's no
            # need to populate the cache just to expire it again.
            related = getattr(instance, self.cache_name, None)

            # If we've got an old related object, we need to clear out its
            # cache. This cache also might not exist if the related object
            # hasn't been accessed yet.
            if related is not None:
                setattr(related, self.field.remote_field.get_cache_name(), None)

            for lh_field, rh_field in self.field.related_fields:
                setattr(instance, lh_field.attname, None)

        # Set the values of the related field.
        else:
            for lh_field, rh_field in self.field.related_fields:
                pk = value._get_pk_val()
                if not self.field.allow_unsaved_instance_assignment and pk is None:
                    raise ValueError(
                        'Cannot assign "%r": "%s" instance isn\'t saved in the database.' %
                        (value, self.field.remote_field.model._meta.object_name)
                    )
                setattr(instance, lh_field.attname, getattr(value, rh_field.attname))

        # Since we already know what the related object is, seed the related
        # object caches now, too. This avoids another db hit if you get the
        # object you just set.
        setattr(instance, self.cache_name, value)
        if value is not None and not self.field.remote_field.multiple:
            setattr(value, self.field.remote_field.get_cache_name(), instance)


def create_foreign_related_manager(superclass, rel):
    """
    Factory function to create a manager that subclasses another manager
    (generally the default manager of a given model) and adds behaviors
    specific to many-to-one relations.
    """

    class RelatedManager(superclass):
        def __init__(self, instance):
            super(RelatedManager, self).__init__()

            self.instance = instance
            self.model = rel.related_model
            self.field = rel.field

            self.core_filters = {self.field.name: instance}

        def __call__(self, **kwargs):
            # We use **kwargs rather than a kwarg argument to enforce the
            # `manager='manager_name'` syntax.
            manager = getattr(self.model, kwargs.pop('manager'))
            manager_class = create_foreign_related_manager(manager.__class__, rel)
            return manager_class(self.instance)
        do_not_call_in_templates = True

        def get_queryset(self):
            try:
                return self.instance._prefetched_objects_cache[self.field.related_query_name()]
            except (AttributeError, KeyError):
                db = self._db or router.db_for_read(self.model, instance=self.instance)
                empty_strings_as_null = connections[db].features.interprets_empty_strings_as_nulls
                qs = super(RelatedManager, self).get_queryset()
                qs._add_hints(instance=self.instance)
                if self._db:
                    qs = qs.using(self._db)
                qs = qs.filter(**self.core_filters)
                for field in self.field.foreign_related_fields:
                    val = getattr(self.instance, field.attname)
                    if val is None or (val == '' and empty_strings_as_null):
                        return qs.none()
                qs._known_related_objects = {self.field: {self.instance.pk: self.instance}}
                return qs

        def get_prefetch_queryset(self, instances, queryset=None):
            if queryset is None:
                queryset = super(RelatedManager, self).get_queryset()

            queryset._add_hints(instance=instances[0])
            queryset = queryset.using(queryset._db or self._db)

            rel_obj_attr = self.field.get_local_related_value
            instance_attr = self.field.get_foreign_related_value
            instances_dict = {instance_attr(inst): inst for inst in instances}
            query = {'%s__in' % self.field.name: instances}
            queryset = queryset.filter(**query)

            # Since we just bypassed this class' get_queryset(), we must manage
            # the reverse relation manually.
            for rel_obj in queryset:
                instance = instances_dict[rel_obj_attr(rel_obj)]
                setattr(rel_obj, self.field.name, instance)
            cache_name = self.field.related_query_name()
            return queryset, rel_obj_attr, instance_attr, False, cache_name

        def add(self, *objs):
            objs = list(objs)
            db = router.db_for_write(self.model, instance=self.instance)
            with transaction.atomic(using=db, savepoint=False):
                for obj in objs:
                    if not isinstance(obj, self.model):
                        raise TypeError("'%s' instance expected, got %r" %
                                        (self.model._meta.object_name, obj))
                    setattr(obj, self.field.name, self.instance)
                    obj.save()
        add.alters_data = True

        def create(self, **kwargs):
            kwargs[self.field.name] = self.instance
            db = router.db_for_write(self.model, instance=self.instance)
            return super(RelatedManager, self.db_manager(db)).create(**kwargs)
        create.alters_data = True

        def get_or_create(self, **kwargs):
            kwargs[self.field.name] = self.instance
            db = router.db_for_write(self.model, instance=self.instance)
            return super(RelatedManager, self.db_manager(db)).get_or_create(**kwargs)
        get_or_create.alters_data = True

        def update_or_create(self, **kwargs):
            kwargs[self.field.name] = self.instance
            db = router.db_for_write(self.model, instance=self.instance)
            return super(RelatedManager, self.db_manager(db)).update_or_create(**kwargs)
        update_or_create.alters_data = True

        # remove() and clear() are only provided if the ForeignKey can have a value of null.
        if rel.field.null:
            def remove(self, *objs, **kwargs):
                if not objs:
                    return
                bulk = kwargs.pop('bulk', True)
                val = self.field.get_foreign_related_value(self.instance)
                old_ids = set()
                for obj in objs:
                    # Is obj actually part of this descriptor set?
                    if self.field.get_local_related_value(obj) == val:
                        old_ids.add(obj.pk)
                    else:
                        raise self.field.remote_field.model.DoesNotExist("%r is not related to %r." % (obj, self.instance))
                self._clear(self.filter(pk__in=old_ids), bulk)
            remove.alters_data = True

            def clear(self, **kwargs):
                bulk = kwargs.pop('bulk', True)
                self._clear(self, bulk)
            clear.alters_data = True

            def _clear(self, queryset, bulk):
                db = router.db_for_write(self.model, instance=self.instance)
                queryset = queryset.using(db)
                if bulk:
                    # `QuerySet.update()` is intrinsically atomic.
                    queryset.update(**{self.field.name: None})
                else:
                    with transaction.atomic(using=db, savepoint=False):
                        for obj in queryset:
                            setattr(obj, self.field.name, None)
                            obj.save(update_fields=[self.field.name])
            _clear.alters_data = True

        def set(self, objs, **kwargs):
            # Force evaluation of `objs` in case it's a queryset whose value
            # could be affected by `manager.clear()`. Refs #19816.
            objs = tuple(objs)

            clear = kwargs.pop('clear', False)

            if self.field.null:
                db = router.db_for_write(self.model, instance=self.instance)
                with transaction.atomic(using=db, savepoint=False):
                    if clear:
                        self.clear()
                        self.add(*objs)
                    else:
                        old_objs = set(self.using(db).all())
                        new_objs = []
                        for obj in objs:
                            if obj in old_objs:
                                old_objs.remove(obj)
                            else:
                                new_objs.append(obj)

                        self.remove(*old_objs)
                        self.add(*new_objs)
            else:
                self.add(*objs)
        set.alters_data = True

    return RelatedManager


class ForeignRelatedObjectsDescriptor(object):
    """
    Accessor to the related objects manager on the reverse side of a
    many-to-one relation.

    In the example::

        class Choice(Model):
            poll = ForeignKey(Place, related_name='choices')

    ``poll.choices`` is a ``ForeignRelatedObjectsDescriptor`` instance.
    """

    def __init__(self, rel):
        self.rel = rel
        self.field = rel.field

    @cached_property
    def related_manager_cls(self):
        return create_foreign_related_manager(
            self.rel.related_model._default_manager.__class__,
            self.rel,
        )

    def __get__(self, instance, instance_type=None):
        if instance is None:
            return self

        return self.related_manager_cls(instance)

    def __set__(self, instance, value):
        manager = self.__get__(instance)
        manager.set(value)


def create_many_related_manager(superclass, rel, reverse):
    """
    Factory function to create a manager that subclasses another manager
    (generally the default manager of a given model) and adds behaviors
    specific to many-to-many relations.
    """

    class ManyRelatedManager(superclass):
        def __init__(self, instance=None):
            super(ManyRelatedManager, self).__init__()

            self.instance = instance

            if not reverse:
                self.model = rel.model
                self.query_field_name = rel.field.related_query_name()
                self.prefetch_cache_name = rel.field.name
                self.source_field_name = rel.field.m2m_field_name()
                self.target_field_name = rel.field.m2m_reverse_field_name()
                self.symmetrical = rel.symmetrical
            else:
                self.model = rel.related_model
                self.query_field_name = rel.field.name
                self.prefetch_cache_name = rel.field.related_query_name()
                self.source_field_name = rel.field.m2m_reverse_field_name()
                self.target_field_name = rel.field.m2m_field_name()
                self.symmetrical = False

            self.through = rel.through
            self.reverse = reverse

            self.source_field = self.through._meta.get_field(self.source_field_name)
            self.target_field = self.through._meta.get_field(self.target_field_name)

            self.core_filters = {}
            for lh_field, rh_field in self.source_field.related_fields:
                self.core_filters['%s__%s' % (self.query_field_name, rh_field.name)] = getattr(instance, rh_field.attname)

            self.related_val = self.source_field.get_foreign_related_value(instance)
            if None in self.related_val:
                raise ValueError('"%r" needs to have a value for field "%s" before '
                                 'this many-to-many relationship can be used.' %
                                 (instance, self.source_field_name))
            # Even if this relation is not to pk, we require still pk value.
            # The wish is that the instance has been already saved to DB,
            # although having a pk value isn't a guarantee of that.
            if instance.pk is None:
                raise ValueError("%r instance needs to have a primary key value before "
                                 "a many-to-many relationship can be used." %
                                 instance.__class__.__name__)

        def __call__(self, **kwargs):
            # We use **kwargs rather than a kwarg argument to enforce the
            # `manager='manager_name'` syntax.
            manager = getattr(self.model, kwargs.pop('manager'))
            manager_class = create_many_related_manager(manager.__class__, rel, reverse)
            return manager_class(instance=self.instance)
        do_not_call_in_templates = True

        def _build_remove_filters(self, removed_vals):
            filters = Q(**{self.source_field_name: self.related_val})
            # No need to add a subquery condition if removed_vals is a QuerySet without
            # filters.
            removed_vals_filters = (not isinstance(removed_vals, QuerySet) or
                                    removed_vals._has_filters())
            if removed_vals_filters:
                filters &= Q(**{'%s__in' % self.target_field_name: removed_vals})
            if self.symmetrical:
                symmetrical_filters = Q(**{self.target_field_name: self.related_val})
                if removed_vals_filters:
                    symmetrical_filters &= Q(
                        **{'%s__in' % self.source_field_name: removed_vals})
                filters |= symmetrical_filters
            return filters

        def get_queryset(self):
            try:
                return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
            except (AttributeError, KeyError):
                qs = super(ManyRelatedManager, self).get_queryset()
                qs._add_hints(instance=self.instance)
                if self._db:
                    qs = qs.using(self._db)
                return qs._next_is_sticky().filter(**self.core_filters)

        def get_prefetch_queryset(self, instances, queryset=None):
            if queryset is None:
                queryset = super(ManyRelatedManager, self).get_queryset()

            queryset._add_hints(instance=instances[0])
            queryset = queryset.using(queryset._db or self._db)

            query = {'%s__in' % self.query_field_name: instances}
            queryset = queryset._next_is_sticky().filter(**query)

            # M2M: need to annotate the query in order to get the primary model
            # that the secondary model was actually related to. We know that
            # there will already be a join on the join table, so we can just add
            # the select.

            # For non-autocreated 'through' models, can't assume we are
            # dealing with PK values.
            fk = self.through._meta.get_field(self.source_field_name)
            join_table = self.through._meta.db_table
            connection = connections[queryset.db]
            qn = connection.ops.quote_name
            queryset = queryset.extra(select={
                '_prefetch_related_val_%s' % f.attname:
                '%s.%s' % (qn(join_table), qn(f.column)) for f in fk.local_related_fields})
            return (
                queryset,
                lambda result: tuple(
                    getattr(result, '_prefetch_related_val_%s' % f.attname)
                    for f in fk.local_related_fields
                ),
                lambda inst: tuple(getattr(inst, f.attname) for f in fk.foreign_related_fields),
                False,
                self.prefetch_cache_name,
            )

        def add(self, *objs):
            if not rel.through._meta.auto_created:
                opts = self.through._meta
                raise AttributeError(
                    "Cannot use add() on a ManyToManyField which specifies an "
                    "intermediary model. Use %s.%s's Manager instead." %
                    (opts.app_label, opts.object_name)
                )

            db = router.db_for_write(self.through, instance=self.instance)
            with transaction.atomic(using=db, savepoint=False):
                self._add_items(self.source_field_name, self.target_field_name, *objs)

                # If this is a symmetrical m2m relation to self, add the mirror entry in the m2m table
                if self.symmetrical:
                    self._add_items(self.target_field_name, self.source_field_name, *objs)
        add.alters_data = True

        def remove(self, *objs):
            if not rel.through._meta.auto_created:
                opts = self.through._meta
                raise AttributeError(
                    "Cannot use remove() on a ManyToManyField which specifies "
                    "an intermediary model. Use %s.%s's Manager instead." %
                    (opts.app_label, opts.object_name)
                )
            self._remove_items(self.source_field_name, self.target_field_name, *objs)
        remove.alters_data = True

        def clear(self):
            db = router.db_for_write(self.through, instance=self.instance)
            with transaction.atomic(using=db, savepoint=False):
                signals.m2m_changed.send(sender=self.through, action="pre_clear",
                    instance=self.instance, reverse=self.reverse,
                    model=self.model, pk_set=None, using=db)

                filters = self._build_remove_filters(super(ManyRelatedManager, self).get_queryset().using(db))
                self.through._default_manager.using(db).filter(filters).delete()

                signals.m2m_changed.send(sender=self.through, action="post_clear",
                    instance=self.instance, reverse=self.reverse,
                    model=self.model, pk_set=None, using=db)
        clear.alters_data = True

        def set(self, objs, **kwargs):
            if not rel.through._meta.auto_created:
                opts = self.through._meta
                raise AttributeError(
                    "Cannot set values on a ManyToManyField which specifies an "
                    "intermediary model. Use %s.%s's Manager instead." %
                    (opts.app_label, opts.object_name)
                )

            # Force evaluation of `objs` in case it's a queryset whose value
            # could be affected by `manager.clear()`. Refs #19816.
            objs = tuple(objs)

            clear = kwargs.pop('clear', False)

            db = router.db_for_write(self.through, instance=self.instance)
            with transaction.atomic(using=db, savepoint=False):
                if clear:
                    self.clear()
                    self.add(*objs)
                else:
                    old_ids = set(self.using(db).values_list(self.target_field.target_field.attname, flat=True))

                    new_objs = []
                    for obj in objs:
                        fk_val = (self.target_field.get_foreign_related_value(obj)[0]
                            if isinstance(obj, self.model) else obj)

                        if fk_val in old_ids:
                            old_ids.remove(fk_val)
                        else:
                            new_objs.append(obj)

                    self.remove(*old_ids)
                    self.add(*new_objs)
        set.alters_data = True

        def create(self, **kwargs):
            # This check needs to be done here, since we can't later remove this
            # from the method lookup table, as we do with add and remove.
            if not self.through._meta.auto_created:
                opts = self.through._meta
                raise AttributeError(
                    "Cannot use create() on a ManyToManyField which specifies "
                    "an intermediary model. Use %s.%s's Manager instead." %
                    (opts.app_label, opts.object_name)
                )
            db = router.db_for_write(self.instance.__class__, instance=self.instance)
            new_obj = super(ManyRelatedManager, self.db_manager(db)).create(**kwargs)
            self.add(new_obj)
            return new_obj
        create.alters_data = True

        def get_or_create(self, **kwargs):
            db = router.db_for_write(self.instance.__class__, instance=self.instance)
            obj, created = super(ManyRelatedManager, self.db_manager(db)).get_or_create(**kwargs)
            # We only need to add() if created because if we got an object back
            # from get() then the relationship already exists.
            if created:
                self.add(obj)
            return obj, created
        get_or_create.alters_data = True

        def update_or_create(self, **kwargs):
            db = router.db_for_write(self.instance.__class__, instance=self.instance)
            obj, created = super(ManyRelatedManager, self.db_manager(db)).update_or_create(**kwargs)
            # We only need to add() if created because if we got an object back
            # from get() then the relationship already exists.
            if created:
                self.add(obj)
            return obj, created
        update_or_create.alters_data = True

        def _add_items(self, source_field_name, target_field_name, *objs):
            # source_field_name: the PK fieldname in join table for the source object
            # target_field_name: the PK fieldname in join table for the target object
            # *objs - objects to add. Either object instances, or primary keys of object instances.

            # If there aren't any objects, there is nothing to do.
            from django.db.models import Model
            if objs:
                new_ids = set()
                for obj in objs:
                    if isinstance(obj, self.model):
                        if not router.allow_relation(obj, self.instance):
                            raise ValueError(
                                'Cannot add "%r": instance is on database "%s", value is on database "%s"' %
                                (obj, self.instance._state.db, obj._state.db)
                            )
                        fk_val = self.through._meta.get_field(
                            target_field_name).get_foreign_related_value(obj)[0]
                        if fk_val is None:
                            raise ValueError(
                                'Cannot add "%r": the value for field "%s" is None' %
                                (obj, target_field_name)
                            )
                        new_ids.add(fk_val)
                    elif isinstance(obj, Model):
                        raise TypeError(
                            "'%s' instance expected, got %r" %
                            (self.model._meta.object_name, obj)
                        )
                    else:
                        new_ids.add(obj)

                db = router.db_for_write(self.through, instance=self.instance)
                vals = (self.through._default_manager.using(db)
                        .values_list(target_field_name, flat=True)
                        .filter(**{
                            source_field_name: self.related_val[0],
                            '%s__in' % target_field_name: new_ids,
                        }))
                new_ids = new_ids - set(vals)

                with transaction.atomic(using=db, savepoint=False):
                    if self.reverse or source_field_name == self.source_field_name:
                        # Don't send the signal when we are inserting the
                        # duplicate data row for symmetrical reverse entries.
                        signals.m2m_changed.send(sender=self.through, action='pre_add',
                            instance=self.instance, reverse=self.reverse,
                            model=self.model, pk_set=new_ids, using=db)

                    # Add the ones that aren't there already
                    self.through._default_manager.using(db).bulk_create([
                        self.through(**{
                            '%s_id' % source_field_name: self.related_val[0],
                            '%s_id' % target_field_name: obj_id,
                        })
                        for obj_id in new_ids
                    ])

                    if self.reverse or source_field_name == self.source_field_name:
                        # Don't send the signal when we are inserting the
                        # duplicate data row for symmetrical reverse entries.
                        signals.m2m_changed.send(sender=self.through, action='post_add',
                            instance=self.instance, reverse=self.reverse,
                            model=self.model, pk_set=new_ids, using=db)

        def _remove_items(self, source_field_name, target_field_name, *objs):
            # source_field_name: the PK colname in join table for the source object
            # target_field_name: the PK colname in join table for the target object
            # *objs - objects to remove
            if not objs:
                return

            # Check that all the objects are of the right type
            old_ids = set()
            for obj in objs:
                if isinstance(obj, self.model):
                    fk_val = self.target_field.get_foreign_related_value(obj)[0]
                    old_ids.add(fk_val)
                else:
                    old_ids.add(obj)

            db = router.db_for_write(self.through, instance=self.instance)
            with transaction.atomic(using=db, savepoint=False):
                # Send a signal to the other end if need be.
                signals.m2m_changed.send(sender=self.through, action="pre_remove",
                    instance=self.instance, reverse=self.reverse,
                    model=self.model, pk_set=old_ids, using=db)
                target_model_qs = super(ManyRelatedManager, self).get_queryset()
                if target_model_qs._has_filters():
                    old_vals = target_model_qs.using(db).filter(**{
                        '%s__in' % self.target_field.target_field.attname: old_ids})
                else:
                    old_vals = old_ids
                filters = self._build_remove_filters(old_vals)
                self.through._default_manager.using(db).filter(filters).delete()

                signals.m2m_changed.send(sender=self.through, action="post_remove",
                    instance=self.instance, reverse=self.reverse,
                    model=self.model, pk_set=old_ids, using=db)

    return ManyRelatedManager


class ManyRelatedObjectsDescriptor(ForeignRelatedObjectsDescriptor):
    """
    Accessor to the related objects manager on the forward and reverse sides of
    a many-to-many relation.

    In the example::

        class Pizza(Model):
            toppings = ManyToManyField(Topping, related_name='pizzas')

    ``pizza.toppings`` and ``topping.pizzas`` are ManyRelatedObjectsDescriptor
    instances.
    """

    def __init__(self, rel, reverse=False):
        super(ManyRelatedObjectsDescriptor, self).__init__(rel)

        self.reverse = reverse

    @property
    def through(self):
        # through is provided so that you have easy access to the through
        # model (Book.authors.through) for inlines, etc. This is done as
        # a property to ensure that the fully resolved value is returned.
        return self.rel.through

    @cached_property
    def related_manager_cls(self):
        model = self.rel.related_model if self.reverse else self.rel.model
        return create_many_related_manager(
            model._default_manager.__class__,
            self.rel,
            reverse=self.reverse,
        )


class ForeignObjectRel(object):
    """
    Used by ForeignObject to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    # Field flags
    auto_created = True
    concrete = False
    editable = False
    is_relation = True

    # Reverse relations are always nullable (Django can't enforce that a
    # foreign key on the related model points to this model).
    null = True

    def __init__(self, field, to, related_name=None, related_query_name=None,
            limit_choices_to=None, parent_link=False, on_delete=None):
        self.field = field
        self.model = to
        self.related_name = related_name
        self.related_query_name = related_query_name
        self.limit_choices_to = {} if limit_choices_to is None else limit_choices_to
        self.parent_link = parent_link
        self.on_delete = on_delete

        self.symmetrical = False
        self.multiple = True

    # Some of the following cached_properties can't be initialized in
    # __init__ as the field doesn't have its model yet. Calling these methods
    # before field.contribute_to_class() has been called will result in
    # AttributeError
    @property
    def to(self):
        warnings.warn(
            "Usage of ForeignObjectRel.to attribute has been deprecated. "
            "Use the model attribute instead.",
            RemovedInDjango21Warning, 2)
        return self.model

    @cached_property
    def hidden(self):
        return self.is_hidden()

    @cached_property
    def name(self):
        return self.field.related_query_name()

    @property
    def remote_field(self):
        return self.field

    @property
    def target_field(self):
        """
        When filtering against this relation, returns the field on the remote
        model against which the filtering should happen.
        """
        target_fields = self.get_path_info()[-1].target_fields
        if len(target_fields) > 1:
            raise exceptions.FieldError("Can't use target_field for multicolumn relations.")
        return target_fields[0]

    @cached_property
    def related_model(self):
        if not self.field.model:
            raise AttributeError(
                "This property can't be accessed before self.field.contribute_to_class has been called.")
        return self.field.model

    @cached_property
    def many_to_many(self):
        return self.field.many_to_many

    @cached_property
    def many_to_one(self):
        return self.field.one_to_many

    @cached_property
    def one_to_many(self):
        return self.field.many_to_one

    @cached_property
    def one_to_one(self):
        return self.field.one_to_one

    def get_prep_lookup(self, lookup_name, value):
        return self.field.get_prep_lookup(lookup_name, value)

    def get_lookup(self, lookup_name):
        return self.field.get_lookup(lookup_name)

    def get_internal_type(self):
        return self.field.get_internal_type()

    @property
    def db_type(self):
        return self.field.db_type

    def __repr__(self):
        return '<%s: %s.%s>' % (
            type(self).__name__,
            self.related_model._meta.app_label,
            self.related_model._meta.model_name,
        )

    def get_choices(self, include_blank=True, blank_choice=BLANK_CHOICE_DASH,
                    limit_to_currently_related=False):
        """
        Return choices with a default blank choices included, for use as
        SelectField choices for this field.

        Analog of django.db.models.fields.Field.get_choices(), provided
        initially for utilization by RelatedFieldListFilter.
        """
        first_choice = blank_choice if include_blank else []
        queryset = self.related_model._default_manager.all()
        if limit_to_currently_related:
            queryset = queryset.complex_filter(
                {'%s__isnull' % self.related_model._meta.model_name: False}
            )
        lst = [(x._get_pk_val(), smart_text(x)) for x in queryset]
        return first_choice + lst

    def get_db_prep_lookup(self, lookup_type, value, connection, prepared=False):
        # Defer to the actual field definition for db prep
        return self.field.get_db_prep_lookup(lookup_type, value, connection=connection, prepared=prepared)

    def is_hidden(self):
        "Should the related object be hidden?"
        return self.related_name is not None and self.related_name[-1] == '+'

    def get_joining_columns(self):
        return self.field.get_reverse_joining_columns()

    def get_extra_restriction(self, where_class, alias, related_alias):
        return self.field.get_extra_restriction(where_class, related_alias, alias)

    def set_field_name(self):
        """
        Sets the related field's name, this is not available until later stages
        of app loading, so set_field_name is called from
        set_attributes_from_rel()
        """
        # By default foreign object doesn't relate to any remote field (for
        # example custom multicolumn joins currently have no remote field).
        self.field_name = None

    def get_accessor_name(self, model=None):
        # This method encapsulates the logic that decides what name to give an
        # accessor descriptor that retrieves related many-to-one or
        # many-to-many objects. It uses the lower-cased object_name + "_set",
        # but this can be overridden with the "related_name" option.
        # Due to backwards compatibility ModelForms need to be able to provide
        # an alternate model. See BaseInlineFormSet.get_default_prefix().
        opts = model._meta if model else self.related_model._meta
        model = model or self.related_model
        if self.multiple:
            # If this is a symmetrical m2m relation on self, there is no reverse accessor.
            if self.symmetrical and model == self.model:
                return None
        if self.related_name:
            return self.related_name
        if opts.default_related_name:
            return opts.default_related_name % {
                'model_name': opts.model_name.lower(),
                'app_label': opts.app_label.lower(),
            }
        return opts.model_name + ('_set' if self.multiple else '')

    def get_cache_name(self):
        return "_%s_cache" % self.get_accessor_name()

    def get_path_info(self):
        return self.field.get_reverse_path_info()


class ManyToOneRel(ForeignObjectRel):
    """
    Used by the ForeignKey field to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.

    Note: Because we somewhat abuse the Rel objects by using them as reverse
    fields we get the funny situation where
    ``ManyToOneRel.many_to_one == False`` and
    ``ManyToOneRel.one_to_many == True``. This is unfortunate but the actual
    ManyToOneRel class is a private API and there is work underway to turn
    reverse relations into actual fields.
    """

    def __init__(self, field, to, field_name, related_name=None, related_query_name=None,
            limit_choices_to=None, parent_link=False, on_delete=None):
        super(ManyToOneRel, self).__init__(
            field, to,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            parent_link=parent_link,
            on_delete=on_delete,
        )

        self.field_name = field_name

    def get_related_field(self):
        """
        Return the Field in the 'to' object to which this relationship is tied.
        """
        field = self.model._meta.get_field(self.field_name)
        if not field.concrete:
            raise FieldDoesNotExist("No related field named '%s'" %
                    self.field_name)
        return field

    def set_field_name(self):
        self.field_name = self.field_name or self.model._meta.pk.name


class OneToOneRel(ManyToOneRel):
    """
    Used by OneToOneField to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    def __init__(self, field, to, field_name, related_name=None, related_query_name=None,
            limit_choices_to=None, parent_link=False, on_delete=None):
        super(OneToOneRel, self).__init__(
            field, to, field_name,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            parent_link=parent_link,
            on_delete=on_delete,
        )

        self.multiple = False


class ManyToManyRel(ForeignObjectRel):
    """
    Used by ManyToManyField to store information about the relation.

    ``_meta.get_fields()`` returns this class to provide access to the field
    flags for the reverse relation.
    """

    def __init__(self, field, to, related_name=None, related_query_name=None,
            limit_choices_to=None, symmetrical=True, through=None, through_fields=None,
            db_constraint=True):
        super(ManyToManyRel, self).__init__(
            field, to,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
        )

        if through and not db_constraint:
            raise ValueError("Can't supply a through model and db_constraint=False")
        self.through = through

        if through_fields and not through:
            raise ValueError("Cannot specify through_fields without a through model")
        self.through_fields = through_fields

        self.symmetrical = symmetrical
        self.db_constraint = db_constraint

    def get_related_field(self):
        """
        Return the field in the 'to' object to which this relationship is tied.
        Provided for symmetry with ManyToOneRel.
        """
        opts = self.through._meta
        if self.through_fields:
            field = opts.get_field(self.through_fields[0])
        else:
            for field in opts.fields:
                rel = getattr(field, 'remote_field', None)
                if rel and rel.model == self.model:
                    break
        return field.foreign_related_fields[0]


class ForeignObject(RelatedField):
    """
    Abstraction of the ForeignKey relation, supports multi-column relations.
    """

    # Field flags
    many_to_many = False
    many_to_one = True
    one_to_many = False
    one_to_one = False

    allow_unsaved_instance_assignment = False
    requires_unique_target = True
    related_accessor_class = ForeignRelatedObjectsDescriptor
    rel_class = ForeignObjectRel

    def __init__(self, to, from_fields, to_fields, rel=None, related_name=None,
            related_query_name=None, limit_choices_to=None, parent_link=False,
            on_delete=CASCADE, swappable=True, **kwargs):
        if rel is None:
            rel = self.rel_class(
                self, to,
                related_name=related_name,
                related_query_name=related_query_name,
                limit_choices_to=limit_choices_to,
                parent_link=parent_link,
                on_delete=on_delete,
            )

        super(ForeignObject, self).__init__(rel=rel, **kwargs)

        self.from_fields = from_fields
        self.to_fields = to_fields
        self.swappable = swappable

    def check(self, **kwargs):
        errors = super(ForeignObject, self).check(**kwargs)
        errors.extend(self._check_unique_target())
        return errors

    def _check_unique_target(self):
        rel_is_string = isinstance(self.remote_field.model, six.string_types)
        if rel_is_string or not self.requires_unique_target:
            return []

        try:
            self.foreign_related_fields
        except FieldDoesNotExist:
            return []

        has_unique_field = any(rel_field.unique
            for rel_field in self.foreign_related_fields)
        if not has_unique_field and len(self.foreign_related_fields) > 1:
            field_combination = ', '.join("'%s'" % rel_field.name
                for rel_field in self.foreign_related_fields)
            model_name = self.remote_field.model.__name__
            return [
                checks.Error(
                    "None of the fields %s on model '%s' have a unique=True constraint."
                    % (field_combination, model_name),
                    hint=None,
                    obj=self,
                    id='fields.E310',
                )
            ]
        elif not has_unique_field:
            field_name = self.foreign_related_fields[0].name
            model_name = self.remote_field.model.__name__
            return [
                checks.Error(
                    ("'%s.%s' must set unique=True "
                     "because it is referenced by a foreign key.") % (model_name, field_name),
                    hint=None,
                    obj=self,
                    id='fields.E311',
                )
            ]
        else:
            return []

    def deconstruct(self):
        name, path, args, kwargs = super(ForeignObject, self).deconstruct()
        kwargs['from_fields'] = self.from_fields
        kwargs['to_fields'] = self.to_fields
        if self.remote_field.related_name is not None:
            kwargs['related_name'] = self.remote_field.related_name
        if self.remote_field.related_query_name is not None:
            kwargs['related_query_name'] = self.remote_field.related_query_name
        if self.remote_field.on_delete != CASCADE:
            kwargs['on_delete'] = self.remote_field.on_delete
        if self.remote_field.parent_link:
            kwargs['parent_link'] = self.remote_field.parent_link
        # Work out string form of "to"
        if isinstance(self.remote_field.model, six.string_types):
            kwargs['to'] = self.remote_field.model
        else:
            kwargs['to'] = "%s.%s" % (self.remote_field.model._meta.app_label, self.remote_field.model._meta.object_name)
        # If swappable is True, then see if we're actually pointing to the target
        # of a swap.
        swappable_setting = self.swappable_setting
        if swappable_setting is not None:
            # If it's already a settings reference, error
            if hasattr(kwargs['to'], "setting_name"):
                if kwargs['to'].setting_name != swappable_setting:
                    raise ValueError(
                        "Cannot deconstruct a ForeignKey pointing to a model "
                        "that is swapped in place of more than one model (%s and %s)"
                        % (kwargs['to'].setting_name, swappable_setting)
                    )
            # Set it
            from django.db.migrations.writer import SettingsReference
            kwargs['to'] = SettingsReference(
                kwargs['to'],
                swappable_setting,
            )
        return name, path, args, kwargs

    def resolve_related_fields(self):
        if len(self.from_fields) < 1 or len(self.from_fields) != len(self.to_fields):
            raise ValueError('Foreign Object from and to fields must be the same non-zero length')
        if isinstance(self.remote_field.model, six.string_types):
            raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
        related_fields = []
        for index in range(len(self.from_fields)):
            from_field_name = self.from_fields[index]
            to_field_name = self.to_fields[index]
            from_field = (self if from_field_name == 'self'
                          else self.opts.get_field(from_field_name))
            to_field = (self.remote_field.model._meta.pk if to_field_name is None
                        else self.remote_field.model._meta.get_field(to_field_name))
            related_fields.append((from_field, to_field))
        return related_fields

    @property
    def related_fields(self):
        if not hasattr(self, '_related_fields'):
            self._related_fields = self.resolve_related_fields()
        return self._related_fields

    @property
    def reverse_related_fields(self):
        return [(rhs_field, lhs_field) for lhs_field, rhs_field in self.related_fields]

    @property
    def local_related_fields(self):
        return tuple(lhs_field for lhs_field, rhs_field in self.related_fields)

    @property
    def foreign_related_fields(self):
        return tuple(rhs_field for lhs_field, rhs_field in self.related_fields)

    def get_local_related_value(self, instance):
        return self.get_instance_value_for_fields(instance, self.local_related_fields)

    def get_foreign_related_value(self, instance):
        return self.get_instance_value_for_fields(instance, self.foreign_related_fields)

    @staticmethod
    def get_instance_value_for_fields(instance, fields):
        ret = []
        opts = instance._meta
        for field in fields:
            # Gotcha: in some cases (like fixture loading) a model can have
            # different values in parent_ptr_id and parent's id. So, use
            # instance.pk (that is, parent_ptr_id) when asked for instance.id.
            if field.primary_key:
                possible_parent_link = opts.get_ancestor_link(field.model)
                if (not possible_parent_link or
                        possible_parent_link.primary_key or
                        possible_parent_link.model._meta.abstract):
                    ret.append(instance.pk)
                    continue
            ret.append(getattr(instance, field.attname))
        return tuple(ret)

    def get_attname_column(self):
        attname, column = super(ForeignObject, self).get_attname_column()
        return attname, None

    def get_joining_columns(self, reverse_join=False):
        source = self.reverse_related_fields if reverse_join else self.related_fields
        return tuple((lhs_field.column, rhs_field.column) for lhs_field, rhs_field in source)

    def get_reverse_joining_columns(self):
        return self.get_joining_columns(reverse_join=True)

    def get_extra_descriptor_filter(self, instance):
        """
        Return an extra filter condition for related object fetching when
        user does 'instance.fieldname', that is the extra filter is used in
        the descriptor of the field.

        The filter should be either a dict usable in .filter(**kwargs) call or
        a Q-object. The condition will be ANDed together with the relation's
        joining columns.

        A parallel method is get_extra_restriction() which is used in
        JOIN and subquery conditions.
        """
        return {}

    def get_extra_restriction(self, where_class, alias, related_alias):
        """
        Return a pair condition used for joining and subquery pushdown. The
        condition is something that responds to as_sql(compiler, connection)
        method.

        Note that currently referring both the 'alias' and 'related_alias'
        will not work in some conditions, like subquery pushdown.

        A parallel method is get_extra_descriptor_filter() which is used in
        instance.fieldname related object fetching.
        """
        return None

    def get_path_info(self):
        """
        Get path from this field to the related model.
        """
        opts = self.remote_field.model._meta
        from_opts = self.model._meta
        return [PathInfo(from_opts, opts, self.foreign_related_fields, self, False, True)]

    def get_reverse_path_info(self):
        """
        Get path from the related model to this field's model.
        """
        opts = self.model._meta
        from_opts = self.remote_field.model._meta
        pathinfos = [PathInfo(from_opts, opts, (opts.pk,), self.remote_field, not self.unique, False)]
        return pathinfos

    def get_lookup(self, lookup_name):
        if lookup_name == 'in':
            return RelatedIn
        elif lookup_name == 'exact':
            return RelatedExact
        elif lookup_name == 'gt':
            return RelatedGreaterThan
        elif lookup_name == 'gte':
            return RelatedGreaterThanOrEqual
        elif lookup_name == 'lt':
            return RelatedLessThan
        elif lookup_name == 'lte':
            return RelatedLessThanOrEqual
        elif lookup_name != 'isnull':
            raise TypeError('Related Field got invalid lookup: %s' % lookup_name)
        return super(ForeignObject, self).get_lookup(lookup_name)

    def get_transform(self, *args, **kwargs):
        raise NotImplementedError('Relational fields do not support transforms.')

    @property
    def attnames(self):
        return tuple(field.attname for field in self.local_related_fields)

    def get_defaults(self):
        return tuple(field.get_default() for field in self.local_related_fields)

    def contribute_to_class(self, cls, name, virtual_only=False):
        super(ForeignObject, self).contribute_to_class(cls, name, virtual_only=virtual_only)
        setattr(cls, self.name, ReverseSingleRelatedObjectDescriptor(self))

    def contribute_to_related_class(self, cls, related):
        # Internal FK's - i.e., those with a related name ending with '+' -
        # and swapped models don't get a related descriptor.
        if not self.remote_field.is_hidden() and not related.related_model._meta.swapped:
            setattr(cls, related.get_accessor_name(), self.related_accessor_class(related))
            # While 'limit_choices_to' might be a callable, simply pass
            # it along for later - this is too early because it's still
            # model load time.
            if self.remote_field.limit_choices_to:
                cls._meta.related_fkey_lookups.append(self.remote_field.limit_choices_to)


class ForeignKey(ForeignObject):
    """
    Provide a many-to-one relation by adding a column to the local model
    to hold the remote value.

    By default ForeignKey will target the pk of the remote model but this
    behavior can be changed by using the ``to_field`` argument.
    """

    # Field flags
    many_to_many = False
    many_to_one = True
    one_to_many = False
    one_to_one = False

    rel_class = ManyToOneRel

    empty_strings_allowed = False
    default_error_messages = {
        'invalid': _('%(model)s instance with %(field)s %(value)r does not exist.')
    }
    description = _("Foreign Key (type determined by related field)")

    def __init__(self, to, to_field=None, related_name=None, related_query_name=None,
            limit_choices_to=None, parent_link=False, on_delete=CASCADE,
            db_constraint=True, **kwargs):
        try:
            to._meta.model_name
        except AttributeError:
            assert isinstance(to, six.string_types), (
                "%s(%r) is invalid. First parameter to ForeignKey must be "
                "either a model, a model name, or the string %r" % (
                    self.__class__.__name__, to,
                    RECURSIVE_RELATIONSHIP_CONSTANT,
                )
            )
        else:
            # For backwards compatibility purposes, we need to *try* and set
            # the to_field during FK construction. It won't be guaranteed to
            # be correct until contribute_to_class is called. Refs #12190.
            to_field = to_field or (to._meta.pk and to._meta.pk.name)

        kwargs['rel'] = self.rel_class(
            self, to, to_field,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            parent_link=parent_link,
            on_delete=on_delete,
        )

        kwargs['db_index'] = kwargs.get('db_index', True)

        super(ForeignKey, self).__init__(
            to, from_fields=['self'], to_fields=[to_field], **kwargs)

        self.db_constraint = db_constraint

    def check(self, **kwargs):
        errors = super(ForeignKey, self).check(**kwargs)
        errors.extend(self._check_on_delete())
        errors.extend(self._check_unique())
        return errors

    def _check_on_delete(self):
        on_delete = getattr(self.remote_field, 'on_delete', None)
        if on_delete == SET_NULL and not self.null:
            return [
                checks.Error(
                    'Field specifies on_delete=SET_NULL, but cannot be null.',
                    hint='Set null=True argument on the field, or change the on_delete rule.',
                    obj=self,
                    id='fields.E320',
                )
            ]
        elif on_delete == SET_DEFAULT and not self.has_default():
            return [
                checks.Error(
                    'Field specifies on_delete=SET_DEFAULT, but has no default value.',
                    hint='Set a default value, or change the on_delete rule.',
                    obj=self,
                    id='fields.E321',
                )
            ]
        else:
            return []

    def _check_unique(self, **kwargs):
        return [
            checks.Warning(
                'Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.',
                hint='ForeignKey(unique=True) is usually better served by a OneToOneField.',
                obj=self,
                id='fields.W342',
            )
        ] if self.unique else []

    def deconstruct(self):
        name, path, args, kwargs = super(ForeignKey, self).deconstruct()
        del kwargs['to_fields']
        del kwargs['from_fields']
        # Handle the simpler arguments
        if self.db_index:
            del kwargs['db_index']
        else:
            kwargs['db_index'] = False
        if self.db_constraint is not True:
            kwargs['db_constraint'] = self.db_constraint
        # Rel needs more work.
        to_meta = getattr(self.remote_field.model, "_meta", None)
        if self.remote_field.field_name and (not to_meta or (to_meta.pk and self.remote_field.field_name != to_meta.pk.name)):
            kwargs['to_field'] = self.remote_field.field_name
        return name, path, args, kwargs

    @property
    def target_field(self):
        return self.foreign_related_fields[0]

    def get_reverse_path_info(self):
        """
        Get path from the related model to this field's model.
        """
        opts = self.model._meta
        from_opts = self.remote_field.model._meta
        pathinfos = [PathInfo(from_opts, opts, (opts.pk,), self.remote_field, not self.unique, False)]
        return pathinfos

    def validate(self, value, model_instance):
        if self.remote_field.parent_link:
            return
        super(ForeignKey, self).validate(value, model_instance)
        if value is None:
            return

        using = router.db_for_read(model_instance.__class__, instance=model_instance)
        qs = self.remote_field.model._default_manager.using(using).filter(
            **{self.remote_field.field_name: value}
        )
        qs = qs.complex_filter(self.get_limit_choices_to())
        if not qs.exists():
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={
                    'model': self.remote_field.model._meta.verbose_name, 'pk': value,
                    'field': self.remote_field.field_name, 'value': value,
                },  # 'pk' is included for backwards compatibility
            )

    def get_attname(self):
        return '%s_id' % self.name

    def get_attname_column(self):
        attname = self.get_attname()
        column = self.db_column or attname
        return attname, column

    def get_default(self):
        "Here we check if the default value is an object and return the to_field if so."
        field_default = super(ForeignKey, self).get_default()
        if isinstance(field_default, self.remote_field.model):
            return getattr(field_default, self.target_field.attname)
        return field_default

    def get_db_prep_save(self, value, connection):
        if value is None or (value == '' and
                             (not self.target_field.empty_strings_allowed or
                              connection.features.interprets_empty_strings_as_nulls)):
            return None
        else:
            return self.target_field.get_db_prep_save(value, connection=connection)

    def get_db_prep_value(self, value, connection, prepared=False):
        return self.target_field.get_db_prep_value(value, connection, prepared)

    def value_to_string(self, obj):
        if not obj:
            # In required many-to-one fields with only one available choice,
            # select that one available choice. Note: For SelectFields
            # we have to check that the length of choices is *2*, not 1,
            # because SelectFields always have an initial "blank" value.
            if not self.blank and self.choices:
                choice_list = self.get_choices_default()
                if len(choice_list) == 2:
                    return smart_text(choice_list[1][0])
        return super(ForeignKey, self).value_to_string(obj)

    def contribute_to_related_class(self, cls, related):
        super(ForeignKey, self).contribute_to_related_class(cls, related)
        if self.remote_field.field_name is None:
            self.remote_field.field_name = cls._meta.pk.name

    def formfield(self, **kwargs):
        db = kwargs.pop('using', None)
        if isinstance(self.remote_field.model, six.string_types):
            raise ValueError("Cannot create form field for %r yet, because "
                             "its related model %r has not been loaded yet" %
                             (self.name, self.remote_field.model))
        defaults = {
            'form_class': forms.ModelChoiceField,
            'queryset': self.remote_field.model._default_manager.using(db),
            'to_field_name': self.remote_field.field_name,
        }
        defaults.update(kwargs)
        return super(ForeignKey, self).formfield(**defaults)

    def db_type(self, connection):
        # The database column type of a ForeignKey is the column type
        # of the field to which it points. An exception is if the ForeignKey
        # points to an AutoField/PositiveIntegerField/PositiveSmallIntegerField,
        # in which case the column type is simply that of an IntegerField.
        # If the database needs similar types for key fields however, the only
        # thing we can do is making AutoField an IntegerField.
        rel_field = self.target_field
        if (isinstance(rel_field, AutoField) or
                (not connection.features.related_fields_match_type and
                isinstance(rel_field, (PositiveIntegerField,
                                       PositiveSmallIntegerField)))):
            return IntegerField().db_type(connection=connection)
        return rel_field.db_type(connection=connection)

    def db_parameters(self, connection):
        return {"type": self.db_type(connection), "check": []}

    def convert_empty_strings(self, value, expression, connection, context):
        if (not value) and isinstance(value, six.string_types):
            return None
        return value

    def get_db_converters(self, connection):
        converters = super(ForeignKey, self).get_db_converters(connection)
        if connection.features.interprets_empty_strings_as_nulls:
            converters += [self.convert_empty_strings]
        return converters

    def get_col(self, alias, output_field=None):
        return super(ForeignKey, self).get_col(alias, output_field or self.target_field)


class OneToOneField(ForeignKey):
    """
    A OneToOneField is essentially the same as a ForeignKey, with the exception
    that it always carries a "unique" constraint with it and the reverse
    relation always returns the object pointed to (since there will only ever
    be one), rather than returning a list.
    """

    # Field flags
    many_to_many = False
    many_to_one = False
    one_to_many = False
    one_to_one = True

    related_accessor_class = SingleRelatedObjectDescriptor
    rel_class = OneToOneRel

    description = _("One-to-one relationship")

    def __init__(self, to, to_field=None, **kwargs):
        kwargs['unique'] = True
        super(OneToOneField, self).__init__(to, to_field, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super(OneToOneField, self).deconstruct()
        if "unique" in kwargs:
            del kwargs['unique']
        return name, path, args, kwargs

    def formfield(self, **kwargs):
        if self.remote_field.parent_link:
            return None
        return super(OneToOneField, self).formfield(**kwargs)

    def save_form_data(self, instance, data):
        if isinstance(data, self.remote_field.model):
            setattr(instance, self.name, data)
        else:
            setattr(instance, self.attname, data)

    def _check_unique(self, **kwargs):
        # Override ForeignKey since check isn't applicable here.
        return []


def create_many_to_many_intermediary_model(field, klass):
    from django.db import models

    def set_managed(model, related, through):
        through._meta.managed = model._meta.managed or related._meta.managed

    to_model = resolve_relation(klass, field.remote_field.model)
    name = '%s_%s' % (klass._meta.object_name, field.name)
    lazy_related_operation(set_managed, klass, to_model, name)

    to = make_model_tuple(to_model)[1]
    from_ = klass._meta.model_name
    if to == from_:
        to = 'to_%s' % to
        from_ = 'from_%s' % from_

    meta = type(str('Meta'), (object,), {
        'db_table': field._get_m2m_db_table(klass._meta),
        'auto_created': klass,
        'app_label': klass._meta.app_label,
        'db_tablespace': klass._meta.db_tablespace,
        'unique_together': (from_, to),
        'verbose_name': '%(from)s-%(to)s relationship' % {'from': from_, 'to': to},
        'verbose_name_plural': '%(from)s-%(to)s relationships' % {'from': from_, 'to': to},
        'apps': field.model._meta.apps,
    })
    # Construct and return the new class.
    return type(str(name), (models.Model,), {
        'Meta': meta,
        '__module__': klass.__module__,
        from_: models.ForeignKey(
            klass,
            related_name='%s+' % name,
            db_tablespace=field.db_tablespace,
            db_constraint=field.remote_field.db_constraint,
        ),
        to: models.ForeignKey(
            to_model,
            related_name='%s+' % name,
            db_tablespace=field.db_tablespace,
            db_constraint=field.remote_field.db_constraint,
        )
    })


class ManyToManyField(RelatedField):
    """
    Provide a many-to-many relation by using an intermediary model that
    holds two ForeignKey fields pointed at the two sides of the relation.

    Unless a ``through`` model was provided, ManyToManyField will use the
    create_many_to_many_intermediary_model factory to automatically generate
    the intermediary model.
    """

    # Field flags
    many_to_many = True
    many_to_one = False
    one_to_many = False
    one_to_one = False

    rel_class = ManyToManyRel

    description = _("Many-to-many relationship")

    def __init__(self, to, related_name=None, related_query_name=None,
            limit_choices_to=None, symmetrical=None, through=None,
            through_fields=None, db_constraint=True, db_table=None,
            swappable=True, **kwargs):
        try:
            to._meta
        except AttributeError:
            assert isinstance(to, six.string_types), (
                "%s(%r) is invalid. First parameter to ManyToManyField must be "
                "either a model, a model name, or the string %r" %
                (self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
            )
            # Class names must be ASCII in Python 2.x, so we forcibly coerce it
            # here to break early if there's a problem.
            to = str(to)

        if symmetrical is None:
            symmetrical = (to == RECURSIVE_RELATIONSHIP_CONSTANT)

        if through is not None:
            assert db_table is None, (
                "Cannot specify a db_table if an intermediary model is used."
            )

        kwargs['rel'] = self.rel_class(
            self, to,
            related_name=related_name,
            related_query_name=related_query_name,
            limit_choices_to=limit_choices_to,
            symmetrical=symmetrical,
            through=through,
            through_fields=through_fields,
            db_constraint=db_constraint,
        )
        self.has_null_arg = 'null' in kwargs

        super(ManyToManyField, self).__init__(**kwargs)

        self.db_table = db_table
        self.swappable = swappable
        # Many-to-many fields are always nullable.
        self.null = True

    def check(self, **kwargs):
        errors = super(ManyToManyField, self).check(**kwargs)
        errors.extend(self._check_unique(**kwargs))
        errors.extend(self._check_relationship_model(**kwargs))
        errors.extend(self._check_ignored_options(**kwargs))
        return errors

    def _check_unique(self, **kwargs):
        if self.unique:
            return [
                checks.Error(
                    'ManyToManyFields cannot be unique.',
                    hint=None,
                    obj=self,
                    id='fields.E330',
                )
            ]
        return []

    def _check_ignored_options(self, **kwargs):
        warnings = []

        if self.has_null_arg:
            warnings.append(
                checks.Warning(
                    'null has no effect on ManyToManyField.',
                    hint=None,
                    obj=self,
                    id='fields.W340',
                )
            )

        if len(self._validators) > 0:
            warnings.append(
                checks.Warning(
                    'ManyToManyField does not support validators.',
                    hint=None,
                    obj=self,
                    id='fields.W341',
                )
            )

        return warnings

    def _check_relationship_model(self, from_model=None, **kwargs):
        if hasattr(self.remote_field.through, '_meta'):
            qualified_model_name = "%s.%s" % (
                self.remote_field.through._meta.app_label, self.remote_field.through.__name__)
        else:
            qualified_model_name = self.remote_field.through

        errors = []

        if self.remote_field.through not in apps.get_models(include_auto_created=True):
            # The relationship model is not installed.
            errors.append(
                checks.Error(
                    ("Field specifies a many-to-many relation through model "
                     "'%s', which has not been installed.") %
                    qualified_model_name,
                    hint=None,
                    obj=self,
                    id='fields.E331',
                )
            )

        else:

            assert from_model is not None, (
                "ManyToManyField with intermediate "
                "tables cannot be checked if you don't pass the model "
                "where the field is attached to."
            )

            # Set some useful local variables
            to_model = resolve_relation(from_model, self.remote_field.model)
            from_model_name = from_model._meta.object_name
            if isinstance(to_model, six.string_types):
                to_model_name = to_model
            else:
                to_model_name = to_model._meta.object_name
            relationship_model_name = self.remote_field.through._meta.object_name
            self_referential = from_model == to_model

            # Check symmetrical attribute.
            if (self_referential and self.remote_field.symmetrical and
                    not self.remote_field.through._meta.auto_created):
                errors.append(
                    checks.Error(
                        'Many-to-many fields with intermediate tables must not be symmetrical.',
                        hint=None,
                        obj=self,
                        id='fields.E332',
                    )
                )

            # Count foreign keys in intermediate model
            if self_referential:
                seen_self = sum(from_model == getattr(field.remote_field, 'model', None)
                    for field in self.remote_field.through._meta.fields)

                if seen_self > 2 and not self.remote_field.through_fields:
                    errors.append(
                        checks.Error(
                            ("The model is used as an intermediate model by "
                             "'%s', but it has more than two foreign keys "
                             "to '%s', which is ambiguous. You must specify "
                             "which two foreign keys Django should use via the "
                             "through_fields keyword argument.") % (self, from_model_name),
                            hint=("Use through_fields to specify which two "
                                  "foreign keys Django should use."),
                            obj=self.remote_field.through,
                            id='fields.E333',
                        )
                    )

            else:
                # Count foreign keys in relationship model
                seen_from = sum(from_model == getattr(field.remote_field, 'model', None)
                    for field in self.remote_field.through._meta.fields)
                seen_to = sum(to_model == getattr(field.remote_field, 'model', None)
                    for field in self.remote_field.through._meta.fields)

                if seen_from > 1 and not self.remote_field.through_fields:
                    errors.append(
                        checks.Error(
                            ("The model is used as an intermediate model by "
                             "'%s', but it has more than one foreign key "
                             "from '%s', which is ambiguous. You must specify "
                             "which foreign key Django should use via the "
                             "through_fields keyword argument.") % (self, from_model_name),
                            hint=('If you want to create a recursive relationship, '
                                  'use ForeignKey("self", symmetrical=False, '
                                  'through="%s").') % relationship_model_name,
                            obj=self,
                            id='fields.E334',
                        )
                    )

                if seen_to > 1 and not self.remote_field.through_fields:
                    errors.append(
                        checks.Error(
                            ("The model is used as an intermediate model by "
                             "'%s', but it has more than one foreign key "
                             "to '%s', which is ambiguous. You must specify "
                             "which foreign key Django should use via the "
                             "through_fields keyword argument.") % (self, to_model_name),
                            hint=('If you want to create a recursive '
                                  'relationship, use ForeignKey("self", '
                                  'symmetrical=False, through="%s").') % relationship_model_name,
                            obj=self,
                            id='fields.E335',
                        )
                    )

                if seen_from == 0 or seen_to == 0:
                    errors.append(
                        checks.Error(
                            ("The model is used as an intermediate model by "
                             "'%s', but it does not have a foreign key to '%s' or '%s'.") % (
                                self, from_model_name, to_model_name
                            ),
                            hint=None,
                            obj=self.remote_field.through,
                            id='fields.E336',
                        )
                    )

        # Validate `through_fields`.
        if self.remote_field.through_fields is not None:
            # Validate that we're given an iterable of at least two items
            # and that none of them is "falsy".
            if not (len(self.remote_field.through_fields) >= 2 and
                    self.remote_field.through_fields[0] and self.remote_field.through_fields[1]):
                errors.append(
                    checks.Error(
                        ("Field specifies 'through_fields' but does not "
                         "provide the names of the two link fields that should be "
                         "used for the relation through model "
                         "'%s'.") % qualified_model_name,
                        hint=("Make sure you specify 'through_fields' as "
                              "through_fields=('field1', 'field2')"),
                        obj=self,
                        id='fields.E337',
                    )
                )

            # Validate the given through fields -- they should be actual
            # fields on the through model, and also be foreign keys to the
            # expected models.
            else:
                assert from_model is not None, (
                    "ManyToManyField with intermediate "
                    "tables cannot be checked if you don't pass the model "
                    "where the field is attached to."
                )

                source, through, target = from_model, self.remote_field.through, self.remote_field.model
                source_field_name, target_field_name = self.remote_field.through_fields[:2]

                for field_name, related_model in ((source_field_name, source),
                                                  (target_field_name, target)):

                    possible_field_names = []
                    for f in through._meta.fields:
                        if hasattr(f, 'remote_field') and getattr(f.remote_field, 'model', None) == related_model:
                            possible_field_names.append(f.name)
                    if possible_field_names:
                        hint = ("Did you mean one of the following foreign "
                                "keys to '%s': %s?") % (related_model._meta.object_name,
                                                        ', '.join(possible_field_names))
                    else:
                        hint = None

                    try:
                        field = through._meta.get_field(field_name)
                    except FieldDoesNotExist:
                        errors.append(
                            checks.Error(
                                ("The intermediary model '%s' has no field '%s'.") % (
                                    qualified_model_name, field_name),
                                hint=hint,
                                obj=self,
                                id='fields.E338',
                            )
                        )
                    else:
                        if not (hasattr(field, 'remote_field') and
                                getattr(field.remote_field, 'model', None) == related_model):
                            errors.append(
                                checks.Error(
                                    "'%s.%s' is not a foreign key to '%s'." % (
                                        through._meta.object_name, field_name,
                                        related_model._meta.object_name),
                                    hint=hint,
                                    obj=self,
                                    id='fields.E339',
                                )
                            )

        return errors

    def deconstruct(self):
        name, path, args, kwargs = super(ManyToManyField, self).deconstruct()
        # Handle the simpler arguments.
        del kwargs["null"]
        if self.db_table is not None:
            kwargs['db_table'] = self.db_table
        if self.remote_field.db_constraint is not True:
            kwargs['db_constraint'] = self.remote_field.db_constraint
        if self.remote_field.related_name is not None:
            kwargs['related_name'] = self.remote_field.related_name
        if self.remote_field.related_query_name is not None:
            kwargs['related_query_name'] = self.remote_field.related_query_name
        # Rel needs more work.
        if isinstance(self.remote_field.model, six.string_types):
            kwargs['to'] = self.remote_field.model
        else:
            kwargs['to'] = "%s.%s" % (
                self.remote_field.model._meta.app_label,
                self.remote_field.model._meta.object_name,
            )
        if getattr(self.remote_field, 'through', None) is not None:
            if isinstance(self.remote_field.through, six.string_types):
                kwargs['through'] = self.remote_field.through
            elif not self.remote_field.through._meta.auto_created:
                kwargs['through'] = "%s.%s" % (
                    self.remote_field.through._meta.app_label,
                    self.remote_field.through._meta.object_name,
                )
        # If swappable is True, then see if we're actually pointing to the target
        # of a swap.
        swappable_setting = self.swappable_setting
        if swappable_setting is not None:
            # If it's already a settings reference, error.
            if hasattr(kwargs['to'], "setting_name"):
                if kwargs['to'].setting_name != swappable_setting:
                    raise ValueError(
                        "Cannot deconstruct a ManyToManyField pointing to a "
                        "model that is swapped in place of more than one model "
                        "(%s and %s)" % (kwargs['to'].setting_name, swappable_setting)
                    )

            from django.db.migrations.writer import SettingsReference
            kwargs['to'] = SettingsReference(
                kwargs['to'],
                swappable_setting,
            )
        return name, path, args, kwargs

    def _get_path_info(self, direct=False):
        """
        Called by both direct and indirect m2m traversal.
        """
        pathinfos = []
        int_model = self.remote_field.through
        linkfield1 = int_model._meta.get_field(self.m2m_field_name())
        linkfield2 = int_model._meta.get_field(self.m2m_reverse_field_name())
        if direct:
            join1infos = linkfield1.get_reverse_path_info()
            join2infos = linkfield2.get_path_info()
        else:
            join1infos = linkfield2.get_reverse_path_info()
            join2infos = linkfield1.get_path_info()
        pathinfos.extend(join1infos)
        pathinfos.extend(join2infos)
        return pathinfos

    def get_path_info(self):
        return self._get_path_info(direct=True)

    def get_reverse_path_info(self):
        return self._get_path_info(direct=False)

    def get_choices_default(self):
        return Field.get_choices(self, include_blank=False)

    def _get_m2m_db_table(self, opts):
        """
        Function that can be curried to provide the m2m table name for this
        relation.
        """
        if self.remote_field.through is not None:
            return self.remote_field.through._meta.db_table
        elif self.db_table:
            return self.db_table
        else:
            return utils.truncate_name('%s_%s' % (opts.db_table, self.name),
                                      connection.ops.max_name_length())

    def _get_m2m_attr(self, related, attr):
        """
        Function that can be curried to provide the source accessor or DB
        column name for the m2m table.
        """
        cache_attr = '_m2m_%s_cache' % attr
        if hasattr(self, cache_attr):
            return getattr(self, cache_attr)
        if self.remote_field.through_fields is not None:
            link_field_name = self.remote_field.through_fields[0]
        else:
            link_field_name = None
        for f in self.remote_field.through._meta.fields:
            if (f.is_relation and f.remote_field.model == related.related_model and
                    (link_field_name is None or link_field_name == f.name)):
                setattr(self, cache_attr, getattr(f, attr))
                return getattr(self, cache_attr)

    def _get_m2m_reverse_attr(self, related, attr):
        """
        Function that can be curried to provide the related accessor or DB
        column name for the m2m table.
        """
        cache_attr = '_m2m_reverse_%s_cache' % attr
        if hasattr(self, cache_attr):
            return getattr(self, cache_attr)
        found = False
        if self.remote_field.through_fields is not None:
            link_field_name = self.remote_field.through_fields[1]
        else:
            link_field_name = None
        for f in self.remote_field.through._meta.fields:
            if f.is_relation and f.remote_field.model == related.model:
                if link_field_name is None and related.related_model == related.model:
                    # If this is an m2m-intermediate to self,
                    # the first foreign key you find will be
                    # the source column. Keep searching for
                    # the second foreign key.
                    if found:
                        setattr(self, cache_attr, getattr(f, attr))
                        break
                    else:
                        found = True
                elif link_field_name is None or link_field_name == f.name:
                    setattr(self, cache_attr, getattr(f, attr))
                    break
        return getattr(self, cache_attr)

    def value_to_string(self, obj):
        data = ''
        if obj:
            qs = getattr(obj, self.name).all()
            data = [instance._get_pk_val() for instance in qs]
        else:
            # In required many-to-many fields with only one available choice,
            # select that one available choice.
            if not self.blank:
                choices_list = self.get_choices_default()
                if len(choices_list) == 1:
                    data = [choices_list[0][0]]
        return smart_text(data)

    def contribute_to_class(self, cls, name, **kwargs):
        # To support multiple relations to self, it's useful to have a non-None
        # related name on symmetrical relations for internal reasons. The
        # concept doesn't make a lot of sense externally ("you want me to
        # specify *what* on my non-reversible relation?!"), so we set it up
        # automatically. The funky name reduces the chance of an accidental
        # clash.
        if self.remote_field.symmetrical and (
                self.remote_field.model == "self" or self.remote_field.model == cls._meta.object_name):
            self.remote_field.related_name = "%s_rel_+" % name
        elif self.remote_field.is_hidden():
            # If the backwards relation is disabled, replace the original
            # related_name with one generated from the m2m field name. Django
            # still uses backwards relations internally and we need to avoid
            # clashes between multiple m2m fields with related_name == '+'.
            self.remote_field.related_name = "_%s_+" % name

        super(ManyToManyField, self).contribute_to_class(cls, name, **kwargs)

        # The intermediate m2m model is not auto created if:
        #  1) There is a manually specified intermediate, or
        #  2) The class owning the m2m field is abstract.
        #  3) The class owning the m2m field has been swapped out.
        if not cls._meta.abstract:
            if self.remote_field.through:
                def resolve_through_model(_, model, field):
                    field.remote_field.through = model
                lazy_related_operation(resolve_through_model, cls, self.remote_field.through, field=self)
            elif not cls._meta.swapped:
                self.remote_field.through = create_many_to_many_intermediary_model(self, cls)

        # Add the descriptor for the m2m relation.
        setattr(cls, self.name, ManyRelatedObjectsDescriptor(self.remote_field, reverse=False))

        # Set up the accessor for the m2m table name for the relation.
        self.m2m_db_table = curry(self._get_m2m_db_table, cls._meta)

    def contribute_to_related_class(self, cls, related):
        # Internal M2Ms (i.e., those with a related name ending with '+')
        # and swapped models don't get a related descriptor.
        if not self.remote_field.is_hidden() and not related.related_model._meta.swapped:
            setattr(cls, related.get_accessor_name(), ManyRelatedObjectsDescriptor(self.remote_field, reverse=True))

        # Set up the accessors for the column names on the m2m table.
        self.m2m_column_name = curry(self._get_m2m_attr, related, 'column')
        self.m2m_reverse_name = curry(self._get_m2m_reverse_attr, related, 'column')

        self.m2m_field_name = curry(self._get_m2m_attr, related, 'name')
        self.m2m_reverse_field_name = curry(self._get_m2m_reverse_attr, related, 'name')

        get_m2m_rel = curry(self._get_m2m_attr, related, 'remote_field')
        self.m2m_target_field_name = lambda: get_m2m_rel().field_name
        get_m2m_reverse_rel = curry(self._get_m2m_reverse_attr, related, 'remote_field')
        self.m2m_reverse_target_field_name = lambda: get_m2m_reverse_rel().field_name

    def set_attributes_from_rel(self):
        pass

    def value_from_object(self, obj):
        """
        Return the value of this field in the given model instance.
        """
        return getattr(obj, self.attname).all()

    def save_form_data(self, instance, data):
        setattr(instance, self.attname, data)

    def formfield(self, **kwargs):
        db = kwargs.pop('using', None)
        defaults = {
            'form_class': forms.ModelMultipleChoiceField,
            'queryset': self.remote_field.model._default_manager.using(db),
        }
        defaults.update(kwargs)
        # If initial is passed in, it's a list of related objects, but the
        # MultipleChoiceField takes a list of IDs.
        if defaults.get('initial') is not None:
            initial = defaults['initial']
            if callable(initial):
                initial = initial()
            defaults['initial'] = [i._get_pk_val() for i in initial]
        return super(ManyToManyField, self).formfield(**defaults)

    def db_type(self, connection):
        # A ManyToManyField is not represented by a single column,
        # so return None.
        return None

    def db_parameters(self, connection):
        return {"type": None, "check": None}
