Form handling with class-based views
====================================

Form processing generally has 3 paths:

* Initial GET (blank or prepopulated form)
* POST with invalid data (typically redisplay form with errors)
* POST with valid data (process the data and typically redirect)

Implementing this yourself often results in a lot of repeated
boilerplate code (see :ref:`Using a form in a
view<using-a-form-in-a-view>`). To help avoid this, Django provides a
collection of generic class-based views for form processing.

Basic Forms
-----------

Given a simple contact form::

    # forms.py
    from django import forms

    class ContactForm(forms.Form):
        name = forms.CharField()
        message = forms.CharField(widget=forms.Textarea)

        def send_email(self):
            # send email using the self.cleaned_data dictionary
            pass

The view can be constructed using a FormView::

    # views.py
    from myapp.forms import ContactForm
    from django.views.generic.edit import FormView

    class ContactView(FormView):
        template_name = 'contact.html'
        form_class = ContactForm
        success_url = '/thanks/'

        def form_valid(self, form):
            # This method is called when valid form data has been POSTed.
            # It should return an HttpResponse.
            form.send_email()
            return super(ContactView, self).form_valid(form)

Notes:

* FormView inherits
  :class:`~django.views.generic.base.TemplateResponseMixin` so
  :attr:`~django.views.generic.base.TemplateResponseMixin.template_name`
  can be used here
* The default implementation for
  :meth:`~django.views.generic.edit.FormView.form_valid` simply
  redirects to the :attr:`success_url`

Model Forms
-----------

Generic views really shine when working with models.  These generic
views will automatically create a :class:`ModelForm`, so long as they
can work out which model class to use:

* If the :attr:`model` attribute is given, that model class will be used
* If :meth:`get_object()` returns an object, the class of that object
  will be used
* If a :attr:`queryset` is given, the model for that queryset will be used

Model form views provide a :meth:`form_valid()` implementation that
saves the model automatically.  You can override this if you have any
special requirements; see below for examples.

You don't even need to provide a attr:`success_url` for
:class:`~django.views.generic.edit.CreateView` or
:class:`~django.views.generic.edit.UpdateView` - they will use
:meth:`get_absolute_url()` on the model object if available.

If you want to use a custom :class:`ModelForm` (for instance to add
extra validation) simply set
:attr:`~django.views.generic.edit.FormMixin.form_class` on your view.

.. note::
    When specifying a custom form class, you must still specify the model,
    even though the :attr:`form_class` may be a :class:`ModelForm`.

First we need to add :meth:`get_absolute_url()` to our :class:`Author`
class:

.. code-block:: python

    # models.py
    from django.core.urlresolvers import reverse
    from django.db import models

    class Author(models.Model):
        name = models.CharField(max_length=200)

        def get_absolute_url(self):
            return reverse('author-detail', kwargs={'pk': self.pk})

Then we can use :class:`CreateView` and friends to do the actual
work. Notice how we're just configuring the generic class-based views
here; we don't have to write any logic ourselves::

    # views.py
    from django.views.generic.edit import CreateView, UpdateView, DeleteView
    from django.core.urlresolvers import reverse_lazy
    from myapp.models import Author

    class AuthorCreate(CreateView):
        model = Author

    class AuthorUpdate(UpdateView):
        model = Author

    class AuthorDelete(DeleteView):
        model = Author
        success_url = reverse_lazy('author-list')

.. note::
    We have to use :func:`~django.core.urlresolvers.reverse_lazy` here, not
    just ``reverse`` as the urls are not loaded when the file is imported.

Finally, we hook these new views into the URLconf::

    # urls.py
    from django.conf.urls import patterns, url
    from myapp.views import AuthorCreate, AuthorUpdate, AuthorDelete

    urlpatterns = patterns('',
        # ...
        url(r'author/add/$', AuthorCreate.as_view(), name='author_add'),
        url(r'author/(?P<pk>\d+)/$', AuthorUpdate.as_view(), name='author_update'),
        url(r'author/(?P<pk>\d+)/delete/$', AuthorDelete.as_view(), name='author_delete'),
    )

.. note::

    These views inherit :class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`
    which uses :attr:`~django.views.generic.detail.SingleObjectTemplateResponseMixin.template_name_prefix`
    to construct the
    :attr:`~django.views.generic.base.TemplateResponseMixin.template_name`
    based on the model.

    In this example:

    * :class:`CreateView` and :class:`UpdateView` use ``myapp/author_form.html``
    * :class:`DeleteView` uses ``myapp/author_confirm_delete.html``

    If you wish to have separate templates for :class:`CreateView` and
    :class:1UpdateView`, you can set either :attr:`template_name` or
    :attr:`template_name_suffix` on your view class.

Models and request.user
-----------------------

To track the user that created an object using a :class:`CreateView`,
you can use a custom :class:`ModelForm` to do this. First, add the
foreign key relation to the model::

    # models.py
    from django.contrib.auth import User
    from django.db import models

    class Author(models.Model):
        name = models.CharField(max_length=200)
        created_by = models.ForeignKey(User)

        # ...

Create a custom :class:`ModelForm` in order to exclude the
``created_by`` field and prevent the user from editing it:

.. code-block:: python

    # forms.py
    from django import forms
    from myapp.models import Author

    class AuthorForm(forms.ModelForm):
        class Meta:
            model = Author
            exclude = ('created_by',)

In the view, use the custom :attr:`form_class` and override
:meth:`form_valid()` to add the user::

    # views.py
    from django.views.generic.edit import CreateView
    from myapp.models import Author
    from myapp.forms import AuthorForm

    class AuthorCreate(CreateView):
        form_class = AuthorForm
        model = Author

        def form_valid(self, form):
            form.instance.created_by = self.request.user
            return super(AuthorCreate, self).form_valid(form)

Note that you'll need to :ref:`decorate this
view<decorating-class-based-views>` using
:func:`~django.contrib.auth.decorators.login_required`, or
alternatively handle unauthorised users in the :meth:`form_valid()`.

AJAX example
------------

Here is a simple example showing how you might go about implementing a form that
works for AJAX requests as well as 'normal' form POSTs::

    import json

    from django.http import HttpResponse
    from django.views.generic.edit import CreateView
    from django.views.generic.detail import SingleObjectTemplateResponseMixin

    class AjaxableResponseMixin(object):
        """
        Mixin to add AJAX support to a form.
        Must be used with an object-based FormView (e.g. CreateView)
        """
        def render_to_json_response(self, context, **response_kwargs):
            data = json.dumps(context)
            response_kwargs['content_type'] = 'application/json'
            return HttpResponse(data, **response_kwargs)

        def form_invalid(self, form):
            if self.request.is_ajax():
                return self.render_to_json_response(form.errors, status=400)
            else:
                return super(AjaxableResponseMixin, self).form_invalid(form)

        def form_valid(self, form):
            if self.request.is_ajax():
                data = {
                    'pk': form.instance.pk,
                }
                return self.render_to_json_response(data)
            else:
                return super(AjaxableResponseMixin, self).form_valid(form)

    class AuthorCreate(AjaxableResponseMixin, CreateView):
        model = Author
