Fixed #6337. Refs #3632 -- Fixed ModelForms subclassing, to the extent that it can be made to work.

This ended up being an almost complete rewrite of ModelForms.__new__, but
should be backwards compatible (although the text of one error message has
changed, which is only user visible and only if you pass in invalid code).

Documentation updated, also.

This started out as a patch from semenov (many thanks!), but by the time all
the problems were hammered out, little of the original was left. Still, it was
a good starting point.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7112 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/newforms/forms.py

@@ -22,12 +22,7 @@ def pretty_name(name):
     name = name[0].upper() + name[1:]
     return name.replace('_', ' ')
 
-class DeclarativeFieldsMetaclass(type):
-    """
-    Metaclass that converts Field attributes to a dictionary called
-    'base_fields', taking into account parent class 'base_fields' as well.
-    """
-    def __new__(cls, name, bases, attrs):
+def get_declared_fields(bases, attrs):
     fields = [(field_name, attrs.pop(field_name)) for field_name, obj in attrs.items() if isinstance(obj, Field)]
     fields.sort(lambda x, y: cmp(x[1].creation_counter, y[1].creation_counter))
 
@@ -38,7 +33,15 @@ class DeclarativeFieldsMetaclass(type):
         if hasattr(base, 'base_fields'):
             fields = base.base_fields.items() + fields
 
-        attrs['base_fields'] = SortedDict(fields)
+    return SortedDict(fields)
+
+class DeclarativeFieldsMetaclass(type):
+    """
+    Metaclass that converts Field attributes to a dictionary called
+    'base_fields', taking into account parent class 'base_fields' as well.
+    """
+    def __new__(cls, name, bases, attrs):
+        attrs['base_fields'] = get_declared_fields(bases, attrs)
         return type.__new__(cls, name, bases, attrs)
 
 class BaseForm(StrAndUnicode):

django/newforms/models.py

@@ -11,7 +11,7 @@ from django.utils.datastructures import SortedDict
 from django.core.exceptions import ImproperlyConfigured
 
 from util import ValidationError, ErrorList
-from forms import BaseForm
+from forms import BaseForm, get_declared_fields
 from fields import Field, ChoiceField, EMPTY_VALUES
 from widgets import Select, SelectMultiple, MultipleHiddenInput
 
@@ -211,57 +211,58 @@ class ModelFormOptions(object):
         self.fields = getattr(options, 'fields', None)
         self.exclude = getattr(options, 'exclude', None)
 
+
 class ModelFormMetaclass(type):
     def __new__(cls, name, bases, attrs,
                 formfield_callback=lambda f: f.formfield()):
-        fields = [(field_name, attrs.pop(field_name)) for field_name, obj in attrs.items() if isinstance(obj, Field)]
-        fields.sort(lambda x, y: cmp(x[1].creation_counter, y[1].creation_counter))
-
-        # If this class is subclassing another Form, add that Form's fields.
-        # Note that we loop over the bases in *reverse*. This is necessary in
-        # order to preserve the correct order of fields.
-        for base in bases[::-1]:
-            if hasattr(base, 'base_fields'):
-                fields = base.base_fields.items() + fields
-        declared_fields = SortedDict(fields)
-
-        opts = ModelFormOptions(attrs.get('Meta', None))
-        attrs['_meta'] = opts
-
-        # Don't allow more than one Meta model definition in bases. The fields
-        # would be generated correctly, but the save method won't deal with
-        # more than one object.
-        base_models = []
-        for base in bases:
+        try:
+            parents = [b for b in bases if issubclass(b, ModelForm)]
+        except NameError:
+            # We are defining ModelForm itself.
+            parents = None
+        if not parents:
+            return super(ModelFormMetaclass, cls).__new__(cls, name, bases,
+                    attrs)
+
+        new_class = type.__new__(cls, name, bases, attrs)
+        declared_fields = get_declared_fields(bases, attrs)
+        opts = new_class._meta = ModelFormOptions(getattr(new_class, 'Meta', None))
+        if opts.model:
+            # If a model is defined, extract form fields from it.
+            fields = fields_for_model(opts.model, opts.fields,
+                                      opts.exclude, formfield_callback)
+            # Fields defined on the base classes override local fields and are
+            # always included.
+            fields.update(declared_fields)
+        else:
+            fields = declared_fields
+        new_class.base_fields = fields
+
+        # XXX: The following is a sanity check for the user to avoid
+        # inadvertent attribute hiding.
+
+        # Search base classes, but don't allow more than one Meta model
+        # definition. The fields would be generated correctly, but the save
+        # method won't deal with more than one object. Also, it wouldn't be
+        # clear what to do with multiple fields and exclude lists.
+        first = None
+        current = opts.model
+        for base in parents:
             base_opts = getattr(base, '_meta', None)
             base_model = getattr(base_opts, 'model', None)
-            if base_model is not None:
-                base_models.append(base_model)
-        if len(base_models) > 1:
+            if base_model:
+                if current:
+                    if base_model is not current:
                         raise ImproperlyConfigured("%s's base classes define more than one model." % name)
-
-        # If a model is defined, extract form fields from it and add them to base_fields
-        if attrs['_meta'].model is not None:
-            # Don't allow a subclass to define a different Meta model than a
-            # parent class has. Technically the right fields would be generated,
-            # but the save method will not deal with more than one model.
-            for base in bases:
-                base_opts = getattr(base, '_meta', None)
-                base_model = getattr(base_opts, 'model', None)
-                if base_model and base_model is not opts.model:
-                    raise ImproperlyConfigured('%s defines a different model than its parent.' % name)
-            model_fields = fields_for_model(opts.model, opts.fields,
-                    opts.exclude, formfield_callback)
-            # fields declared in base classes override fields from the model
-            model_fields.update(declared_fields)
-            attrs['base_fields'] = model_fields
                 else:
-            attrs['base_fields'] = declared_fields
-        return type.__new__(cls, name, bases, attrs)
+                    current = base_model
+
+        return new_class
 
 class BaseModelForm(BaseForm):
     def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
-                 initial=None, error_class=ErrorList, label_suffix=':', instance=None):
+                 initial=None, error_class=ErrorList, label_suffix=':',
+                 instance=None):
         opts = self._meta
         if instance is None:
             # if we didn't get an instance, instantiate a new one
@@ -277,7 +278,8 @@ class BaseModelForm(BaseForm):
 
     def save(self, commit=True):
         """
-        Saves this ``form``'s cleaned_data into model instance ``self.instance``.
+        Saves this ``form``'s cleaned_data into model instance
+        ``self.instance``.
 
         If commit=True, then the changes to ``instance`` will be saved to the
         database. Returns ``instance``.

docs/modelforms.txt

@@ -320,3 +320,41 @@ parameter when declaring the form field::
    ...
    ...     class Meta:
    ...         model = Article
+
+Form inheritance
+----------------
+As with the basic forms, you can extend and reuse ``ModelForms`` by inheriting
+them. Normally, this will be useful if you need to declare some extra fields
+or extra methods on a parent class for use in a number of forms derived from
+models. For example, using the previous ``ArticleForm`` class::
+
+    >>> class EnhancedArticleForm(ArticleForm):
+    ...     def clean_pub_date(self):
+    ...         ...
+
+This creates a form that behaves identically to ``ArticleForm``, except there
+is some extra validation and cleaning for the ``pub_date`` field.
+
+There are a couple of things to note, however. Most of these won't normally be
+of concern unless you are trying to do something tricky with subclassing.
+
+ * All the fields from the parent classes will appear in the child
+   ``ModelForm``. This means you cannot change a parent's ``Meta.exclude``
+   attribute, for example, and except it to have an effect, since the field is
+   already part of the field list in the parent class.
+
+ * Normal Python name resolution rules apply. If you have multiple base
+   classes that declare a ``Meta`` inner class, only the first one will be
+   used.  This means the child's ``Meta``, if it exists, otherwise the
+   ``Meta`` of the first parent, etc.
+
+ * For technical reasons, you cannot have a subclass that is inherited from
+   both a ``ModelForm`` and a ``Form`` simultaneously.
+
+Because of the "child inherits all fields from parents" behaviour, you
+shouldn't try to declare model fields in multiple classes (parent and child).
+Instead, declare all the model-related stuff in one class and use inheritance
+to add "extra" non-model fields and methods to the final result. Whether you
+put the "extra" functions in the parent class or the child class will depend
+on how you intend to reuse them.
+

tests/modeltests/model_forms/models.py

@@ -160,7 +160,7 @@ familiar with the mechanics.
 ...         model = Article
 Traceback (most recent call last):
 ...
-ImproperlyConfigured: BadForm defines a different model than its parent.
+ImproperlyConfigured: BadForm's base classes define more than one model.
 
 >>> class ArticleForm(ModelForm):
 ...     class Meta:
@@ -179,6 +179,20 @@ This one is OK since the subclass specifies the same model as the parent.
 ...         model = Category
 
 
+Subclassing without specifying a Meta on the class will use the parent's Meta
+(or the first parent in the MRO if there are multiple parent classes).
+
+>>> class CategoryForm(ModelForm):
+...     class Meta:
+...         model = Category
+...         exclude = ['url']
+>>> class SubCategoryForm(CategoryForm):
+...     pass
+
+>>> print SubCategoryForm()
+<tr><th><label for="id_name">Name:</label></th><td><input id="id_name" type="text" name="name" maxlength="20" /></td></tr>
+<tr><th><label for="id_slug">Slug:</label></th><td><input id="id_slug" type="text" name="slug" maxlength="20" /></td></tr>
+
 # Old form_for_x tests #######################################################
 
 >>> from django.newforms import ModelForm, CharField