====================================
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 contact form:

.. code-block:: python
    :caption: ``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``:

.. code-block:: python
    :caption: ``views.py``

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


    class ContactFormView(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().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.FormMixin.form_valid` simply
  redirects to the :attr:`~django.views.generic.edit.FormMixin.success_url`.

Model forms
===========

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

* If the :attr:`~django.views.generic.edit.ModelFormMixin.model` attribute is
  given, that model class will be used.
* If :meth:`~django.views.generic.detail.SingleObjectMixin.get_object`
  returns an object, the class of that object will be used.
* If a :attr:`~django.views.generic.detail.SingleObjectMixin.queryset` is
  given, the model for that queryset will be used.

Model form views provide a
:meth:`~django.views.generic.edit.ModelFormMixin.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 ``success_url`` for
:class:`~django.views.generic.edit.CreateView` or
:class:`~django.views.generic.edit.UpdateView` - they will use
:meth:`~django.db.models.Model.get_absolute_url` on the model object if
available.

If you want to use a custom :class:`~django.forms.ModelForm` (for instance to
add extra validation), 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:`~django.views.generic.edit.FormMixin.form_class` may
    be a :class:`~django.forms.ModelForm`.

First we need to add :meth:`~django.db.models.Model.get_absolute_url` to our
``Author`` class:

.. code-block:: python
    :caption: ``models.py``

    from django.db import models
    from django.urls import reverse


    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:

.. code-block:: python
    :caption: ``views.py``

    from django.urls import reverse_lazy
    from django.views.generic.edit import CreateView, DeleteView, UpdateView
    from myapp.models import Author


    class AuthorCreateView(CreateView):
        model = Author
        fields = ["name"]


    class AuthorUpdateView(UpdateView):
        model = Author
        fields = ["name"]


    class AuthorDeleteView(DeleteView):
        model = Author
        success_url = reverse_lazy("author-list")

.. note::
    We have to use :func:`~django.urls.reverse_lazy` instead of
    ``reverse()``, as the urls are not loaded when the file is imported.

The ``fields`` attribute works the same way as the ``fields`` attribute on the
inner ``Meta`` class on :class:`~django.forms.ModelForm`. Unless you define the
form class in another way, the attribute is required and the view will raise
an :exc:`~django.core.exceptions.ImproperlyConfigured` exception if it's not.

If you specify both the
:attr:`~django.views.generic.edit.ModelFormMixin.fields` and
:attr:`~django.views.generic.edit.FormMixin.form_class` attributes, an
:exc:`~django.core.exceptions.ImproperlyConfigured` exception will be raised.

Finally, we hook these new views into the URLconf:

.. code-block:: python
    :caption: ``urls.py``

    from django.urls import path
    from myapp.views import AuthorCreateView, AuthorDeleteView, AuthorUpdateView

    urlpatterns = [
        # ...
        path("author/add/", AuthorCreateView.as_view(), name="author-add"),
        path("author/<int:pk>/", AuthorUpdateView.as_view(), name="author-update"),
        path("author/<int:pk>/delete/", AuthorDeleteView.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_suffix`
    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:`UpdateView`, you can set either
    :attr:`~django.views.generic.base.TemplateResponseMixin.template_name` or
    :attr:`~django.views.generic.detail.SingleObjectTemplateResponseMixin.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:`~django.forms.ModelForm` to do this. First, add
the foreign key relation to the model:

.. code-block:: python
    :caption: ``models.py``

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


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

        # ...

In the view, ensure that you don't include ``created_by`` in the list of fields
to edit, and override
:meth:`~django.views.generic.edit.ModelFormMixin.form_valid` to add the user:

.. code-block:: python
    :caption: ``views.py``

    from django.contrib.auth.mixins import LoginRequiredMixin
    from django.views.generic.edit import CreateView
    from myapp.models import Author


    class AuthorCreateView(LoginRequiredMixin, CreateView):
        model = Author
        fields = ["name"]

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

:class:`~django.contrib.auth.mixins.LoginRequiredMixin` prevents users who
aren't logged in from accessing the form. If you omit that, you'll need to
handle unauthorized users in :meth:`~.ModelFormMixin.form_valid`.

.. _content-negotiation-example:

Content negotiation example
===========================

Here is an example showing how you might go about implementing a form that
works with an API-based workflow as well as 'normal' form POSTs::

    from django.http import JsonResponse
    from django.views.generic.edit import CreateView
    from myapp.models import Author


    class JsonableResponseMixin:
        """
        Mixin to add JSON support to a form.
        Must be used with an object-based FormView (e.g. CreateView)
        """

        def form_invalid(self, form):
            response = super().form_invalid(form)
            if self.request.accepts("text/html"):
                return response
            else:
                return JsonResponse(form.errors, status=400)

        def form_valid(self, form):
            # We make sure to call the parent's form_valid() method because
            # it might do some processing (in the case of CreateView, it will
            # call form.save() for example).
            response = super().form_valid(form)
            if self.request.accepts("text/html"):
                return response
            else:
                data = {
                    "pk": self.object.pk,
                }
                return JsonResponse(data)


    class AuthorCreateView(JsonableResponseMixin, CreateView):
        model = Author
        fields = ["name"]

The above example assumes that if the client supports ``text/html``, that they
would prefer it. However, this may not always be true. When requesting a
``.css`` file, many browsers will send the header
``Accept: text/css,*/*;q=0.1``, indicating that they would prefer CSS, but
anything else is fine. This means ``request.accepts("text/html")`` will be
``True``.

To determine the correct format, taking into consideration the client's
preference, use :func:`django.http.HttpRequest.get_preferred_type`::

    class JsonableResponseMixin:
        """
        Mixin to add JSON support to a form.
        Must be used with an object-based FormView (e.g. CreateView).
        """

        accepted_media_types = ["text/html", "application/json"]

        def dispatch(self, request, *args, **kwargs):
            if request.get_preferred_type(self.accepted_media_types) is None:
                # No format in common.
                return HttpResponse(
                    status_code=406, headers={"Accept": ",".join(self.accepted_media_types)}
                )

            return super().dispatch(request, *args, **kwargs)

        def form_invalid(self, form):
            response = super().form_invalid(form)
            accepted_type = self.request.get_preferred_type(self.accepted_media_types)
            if accepted_type == "text/html":
                return response
            elif accepted_type == "application/json":
                return JsonResponse(form.errors, status=400)

        def form_valid(self, form):
            # We make sure to call the parent's form_valid() method because
            # it might do some processing (in the case of CreateView, it will
            # call form.save() for example).
            response = super().form_valid(form)
            accepted_type = self.request.get_preferred_type(self.accepted_media_types)
            if accepted_type == "text/html":
                return response
            elif accepted_type == "application/json":
                data = {
                    "pk": self.object.pk,
                }
                return JsonResponse(data)
