django/docs/topics/class-based-views/mixins.txt

===================================
Using mixins with class-based views
===================================

.. caution::

    This is an advanced topic. A working knowledge of :doc:`Django's
    class-based views<index>` is advised before exploring these
    techniques.

Django's built-in class-based views provide a lot of functionality,
but some of it you may want to use separately. For instance, you may
want to write a view that renders a template to make the HTTP
response, but you can't use
:class:`~django.views.generic.base.TemplateView`; perhaps you need to
render a template only on `POST`, with `GET` doing something else
entirely. While you could use
:class:`~django.template.response.TemplateResponse` directly, this
will likely result in duplicate code.

For this reason, Django also provides a number of mixins that provide
more discrete functionality. Template rendering, for instance, is
encapsulated in the
:class:`~django.views.generic.base.TemplateResponseMixin`. The Django
reference documentation contains :doc:`full documentation of all the
mixins</ref/class-based-views/mixins>`.

Context and template responses
==============================

Two central mixins are provided that help in providing a consistent
interface to working with templates in class-based views.

:class:`~django.views.generic.base.TemplateResponseMixin`
    Every built in view which returns a
    :class:`~django.template.response.TemplateResponse` will call the
    :meth:`~django.views.generic.base.TemplateResponseMixin.render_to_response`
    method that :class:`TemplateResponseMixin` provides. Most of the time this
    will be called for you (for instance, it is called by the ``get()`` method
    implemented by both :class:`~django.views.generic.base.TemplateView` and
    :class:`~django.views.generic.base.DetailView`); similarly, it's unlikely
    that you'll need to override it, although if you want your response to
    return something not rendered via a Django template then you'll want to do
    it. For an example of this, see the :ref:`JSONResponseMixin example
    <jsonresponsemixin-example>`.

    ``render_to_response`` itself calls
    :meth:`~django.views.generic.base.TemplateResponseMixin.get_template_names`,
    which by default will just look up
    :attr:`~django.views.generic.base.TemplateResponseMixin.template_name` on
    the class-based view; two other mixins
    (:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`
    and
    :class:`~django.views.generic.list.MultipleObjectTemplateResponseMixin`)
    override this to provide more flexible defaults when dealing with actual
    objects.

.. versionadded:: 1.5

:class:`~django.views.generic.base.ContextMixin`
    Every built in view which needs context data, such as for rendering a
    template (including :class:`TemplateResponseMixin` above), should call
    :meth:`~django.views.generic.base.ContextMixin.get_context_data` passing
    any data they want to ensure is in there as keyword arguments.
    ``get_context_data`` returns a dictionary; in :class:`ContextMixin` it
    simply returns its keyword arguments, but it is common to override this to
    add more members to the dictionary.

Building up Django's generic class-based views
==============================================

Let's look at how two of Django's generic class-based views are built
out of mixins providing discrete functionality. We'll consider
:class:`~django.views.generic.detail.DetailView`, which renders a
"detail" view of an object, and
:class:`~django.views.generic.list.ListView`, which will render a list
of objects, typically from a queryset, and optionally paginate
them. This will introduce us to four mixins which between them provide
useful functionality when working with either a single Django object,
or multiple objects.

There are also mixins involved in the generic edit views
(:class:`~django.views.generic.edit.FormView`, and the model-specific
views :class:`~django.views.generic.edit.CreateView`,
:class:`~django.views.generic.edit.UpdateView` and
:class:`~django.views.generic.edit.DeleteView`), and in the
date-based generic views. These are
covered in the :doc:`mixin reference
documentation</ref/class-based-views/mixins>`.

DetailView: working with a single Django object
-----------------------------------------------

To show the detail of an object, we basically need to do two things:
we need to look up the object and then we need to make a
:class:`TemplateResponse` with a suitable template, and that object as
context.

To get the object, :class:`~django.views.generic.detail.DetailView`
relies on :class:`~django.views.generic.detail.SingleObjectMixin`,
which provides a
:meth:`~django.views.generic.detail.SingleObjectMixin.get_object`
method that figures out the object based on the URL of the request (it
looks for ``pk`` and ``slug`` keyword arguments as declared in the
URLConf, and looks the object up either from the
:attr:`~django.views.generic.detail.SingleObjectMixin.model` attribute
on the view, or the
:attr:`~django.views.generic.detail.SingleObjectMixin.queryset`
attribute if that's provided). :class:`SingleObjectMixin` also overrides
:meth:`~django.views.generic.base.ContextMixin.get_context_data`,
which is used across all Django's built in class-based views to supply
context data for template renders.

To then make a :class:`TemplateResponse`, :class:`DetailView` uses
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`,
which extends
:class:`~django.views.generic.base.TemplateResponseMixin`, overriding
:meth:`get_template_names()` as discussed above. It actually provides
a fairly sophisticated set of options, but the main one that most
people are going to use is
``<app_label>/<object_name>_detail.html``. The ``_detail`` part can be
changed by setting
:attr:`~django.views.generic.detail.SingleObjectTemplateResponseMixin.template_name_suffix`
on a subclass to something else. (For instance, the :doc:`generic edit
views<generic-editing>` use ``_form`` for create and update views, and
``_confirm_delete`` for delete views.)

ListView: working with many Django objects
------------------------------------------

Lists of objects follow roughly the same pattern: we need a (possibly
paginated) list of objects, typically a :class:`QuerySet`, and then we need
to make a :class:`TemplateResponse` with a suitable template using
that list of objects.

To get the objects, :class:`~django.views.generic.list.ListView` uses
:class:`~django.views.generic.list.MultipleObjectMixin`, which
provides both
:meth:`~django.views.generic.list.MultipleObjectMixin.get_queryset`
and
:meth:`~django.views.generic.list.MultipleObjectMixin.paginate_queryset`. Unlike
with :class:`SingleObjectMixin`, there's no need to key off parts of
the URL to figure out the queryset to work with, so the default just
uses the
:attr:`~django.views.generic.list.MultipleObjectMixin.queryset` or
:attr:`~django.views.generic.list.MultipleObjectMixin.model` attribute
on the view class. A common reason to override
:meth:`~django.views.generic.list.MultipleObjectMixin.get_queryset`
here would be to dynamically vary the objects, such as depending on
the current user or to exclude posts in the future for a blog.

:class:`MultipleObjectMixin` also overrides
:meth:`~django.views.generic.base.ContextMixin.get_context_data` to
include appropriate context variables for pagination (providing
dummies if pagination is disabled). It relies on ``object_list`` being
passed in as a keyword argument, which :class:`ListView` arranges for
it.

To make a :class:`TemplateResponse`, :class:`ListView` then uses
:class:`~django.views.generic.list.MultipleObjectTemplateResponseMixin`;
as with :class:`SingleObjectTemplateResponseMixin` above, this
overrides :meth:`get_template_names()` to provide :meth:`a range of
options
<~django.views.generic.list.MultipleObjectTempalteResponseMixin>`,
with the most commonly-used being
``<app_label>/<object_name>_list.html``, with the ``_list`` part again
being taken from the
:attr:`~django.views.generic.list.MultipleObjectTemplateResponseMixin.template_name_suffix`
attribute. (The date based generic views use suffixes such as ``_archive``,
``_archive_year`` and so on to use different templates for the various
specialised date-based list views.)

Using Django's class-based view mixins
======================================

Now we've seen how Django's generic class-based views use the provided
mixins, let's look at other ways we can combine them. Of course we're
still going to be combining them with either built-in class-based
views, or other generic class-based views, but there are a range of
rarer problems you can solve than are provided for by Django out of
the box.

.. warning::

    Not all mixins can be used together, and not all generic class
    based views can be used with all other mixins. Here we present a
    few examples that do work; if you want to bring together other
    functionality then you'll have to consider interactions between
    attributes and methods that overlap between the different classes
    you're using, and how `method resolution order`_ will affect which
    versions of the methods will be called in what order.

    The reference documentation for Django's :doc:`class-based
    views</ref/class-based-views/index>` and :doc:`class-based view
    mixins</ref/class-based-views/mixins>` will help you in
    understanding which attributes and methods are likely to cause
    conflict between different classes and mixins.

    If in doubt, it's often better to back off and base your work on
    :class:`View` or :class:`TemplateView`, perhaps with
    :class:`SimpleObjectMixin` and
    :class:`MultipleObjectMixin`. Although you will probably end up
    writing more code, it is more likely to be clearly understandable
    to someone else coming to it later, and with fewer interactions to
    worry about you will save yourself some thinking. (Of course, you
    can always dip into Django's implementation of the generic class
    based views for inspiration on how to tackle problems.)

.. _method resolution order: http://www.python.org/download/releases/2.3/mro/


Using SingleObjectMixin with View
---------------------------------

If we want to write a simple class-based view that responds only to
``POST``, we'll subclass :class:`~django.views.generic.base.View` and
write a ``post()`` method in the subclass. However if we want our
processing to work on a particular object, identified from the URL,
we'll want the functionality provided by
:class:`~django.views.generic.detail.SingleObjectMixin`.

We'll demonstrate this with the publisher modelling we used in the
:doc:`generic class-based views introduction<generic-display>`.

.. code-block:: python

    # views.py
    from django.http import HttpResponseForbidden, HttpResponseRedirect
    from django.core.urlresolvers import reverse
    from django.views.generic import View
    from django.views.generic.detail import SingleObjectMixin
    from books.models import Author

    class RecordInterest(View, SingleObjectMixin):
        """Records the current user's interest in an author."""
        model = Author

        def post(self, request, *args, **kwargs):
            if not request.user.is_authenticated():
                return HttpResponseForbidden()

            # Look up the author we're interested in.
            self.object = self.get_object()
            # Actually record interest somehow here!

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

In practice you'd probably want to record the interest in a key-value
store rather than in a relational database, so we've left that bit
out. The only bit of the view that needs to worry about using
:class:`SingleObjectMixin` is where we want to look up the author
we're interested in, which it just does with a simple call to
``self.get_object()``. Everything else is taken care of for us by the
mixin.

We can hook this into our URLs easily enough::

    # urls.py
    from books.views import RecordInterest

    urlpatterns = patterns('',
        #...
        url(r'^author/(?P<pk>\d+)/interest/$', RecordInterest.as_view(), name='author-interest'),
    )

Note the ``pk`` named group, which
:meth:`~django.views.generic.detail.SingleObjectMixin.get_object` uses
to look up the :class:`Author` instance. You could also use a slug, or
any of the other features of :class:`SingleObjectMixin`.

Using SingleObjectMixin with ListView
-------------------------------------

:class:`~django.views.generic.list.ListView` provides built-in
pagination, but you might want to paginate a list of objects that are
all linked (by a foreign key) to another object. In our publishing
example, you might want to paginate through all the books by a
particular publisher.

One way to do this is to combine :class:`ListView` with
:class:`SingleObjectMixin`, so that the queryset for the paginated
list of books can hang off the publisher found as the single
object. In order to do this, we need to have two different querysets:

**Publisher queryset for use in get_object**
    We'll set that up directly when we call :meth:`get_object()`.

**Book queryset for use by ListView**
    We'll figure that out ourselves in :meth:`get_queryset()` so we
    can take into account the Publisher we're looking at.

.. note::

    We have to think carefully about :meth:`get_context_data()`.
    Since both :class:`SingleObjectMixin` and :class:`ListView` will
    put things in the context data under the value of
    :attr:`context_object_name` if it's set, we'll instead explictly
    ensure the Publisher is in the context data. :class:`ListView`
    will add in the suitable ``page_obj`` and ``paginator`` for us
    providing we remember to call ``super()``.

Now we can write a new :class:`PublisherDetail`::

    from django.views.generic import ListView
    from django.views.generic.detail import SingleObjectMixin
    from books.models import Publisher

    class PublisherDetail(SingleObjectMixin, ListView):
        paginate_by = 2
        template_name = "books/publisher_detail.html"

        def get_context_data(self, **kwargs):
            kwargs['publisher'] = self.object
            return super(PublisherDetail, self).get_context_data(**kwargs)

        def get_queryset(self):
            self.object = self.get_object(Publisher.objects.all())
            return self.object.book_set.all()

Notice how we set ``self.object`` within :meth:`get_queryset` so we
can use it again later in :meth:`get_context_data`. If you don't set
:attr:`template_name`, the template will default to the normal
:class:`ListView` choice, which in this case would be
``"books/book_list.html"`` because it's a list of books;
:class:`ListView` knows nothing about :class:`SingleObjectMixin`, so
it doesn't have any clue this view is anything to do with a Publisher.

.. highlightlang:: html+django

The ``paginate_by`` is deliberately small in the example so you don't
have to create lots of books to see the pagination working! Here's the
template you'd want to use::

    {% extends "base.html" %}

    {% block content %}
        <h2>Publisher {{ publisher.name }}</h2>

        <ol>
          {% for book in page_obj %}
            <li>{{ book.title }}</li>
          {% endfor %}
        </ol>

        <div class="pagination">
            <span class="step-links">
                {% if page_obj.has_previous %}
                    <a href="?page={{ page_obj.previous_page_number }}">previous</a>
                {% endif %}

                <span class="current">
                    Page {{ page_obj.number }} of {{ paginator.num_pages }}.
                </span>

                {% if page_obj.has_next %}
                    <a href="?page={{ page_obj.next_page_number }}">next</a>
                {% endif %}
            </span>
        </div>
    {% endblock %}

Avoid anything more complex
===========================

Generally you can use
:class:`~django.views.generic.base.TemplateResponseMixin` and
:class:`~django.views.generic.detail.SingleObjectMixin` when you need
their functionality. As shown above, with a bit of care you can even
combine :class:`SingleObjectMixin` with
:class:`~django.views.generic.list.ListView`. However things get
increasingly complex as you try to do so, and a good rule of thumb is:

.. hint::

    Each of your views should use only mixins or views from one of the
    groups of generic class-based views: :doc:`detail,
    list<generic-display>`, :doc:`editing<generic-editing>` and
    date. For example it's fine to combine
    :class:`TemplateView` (built in view) with
    :class:`MultipleObjectMixin` (generic list), but you're likely to
    have problems combining :class:`SingleObjectMixin` (generic
    detail) with :class:`MultipleObjectMixin` (generic list).

To show what happens when you try to get more sophisticated, we show
an example that sacrifices readability and maintainability when there
is a simpler solution. First, let's look at a naive attempt to combine
:class:`~django.views.generic.detail.DetailView` with
:class:`~django.views.generic.edit.FormMixin` to enable use to
``POST`` a Django :class:`Form` to the same URL as we're displaying an
object using :class:`DetailView`.

Using FormMixin with DetailView
-------------------------------

Think back to our earlier example of using :class:`View` and
:class:`SingleObjectMixin` together. We were recording a user's
interest in a particular author; say now that we want to let them
leave a message saying why they like them. Again, let's assume we're
not going to store this in a relational database but instead in
something more esoteric that we won't worry about here.

At this point it's natural to reach for a :class:`Form` to encapsulate
the information sent from the user's browser to Django. Say also that
we're heavily invested in `REST`_, so we want to use the same URL for
displaying the author as for capturing the message from the
user. Let's rewrite our :class:`AuthorDetailView` to do that.

.. _REST: http://en.wikipedia.org/wiki/Representational_state_transfer

We'll keep the ``GET`` handling from :class:`DetailView`, although
we'll have to add a :class:`Form` into the context data so we can
render it in the template. We'll also want to pull in form processing
from :class:`~django.views.generic.edit.FormMixin`, and write a bit of
code so that on ``POST`` the form gets called appropriately.

.. note::

    We use :class:`FormMixin` and implement :meth:`post()` ourselves
    rather than try to mix :class:`DetailView` with :class:`FormView`
    (which provides a suitable :meth:`post()` already) because both of
    the views implement :meth:`get()`, and things would get much more
    confusing.

.. highlightlang:: python

Our new :class:`AuthorDetail` looks like this::

    # CAUTION: you almost certainly do not want to do this.
    # It is provided as part of a discussion of problems you can
    # run into when combining different generic class-based view
    # functionality that is not designed to be used together.

    from django import forms
    from django.http import HttpResponseForbidden
    from django.core.urlresolvers import reverse
    from django.views.generic import DetailView
    from django.views.generic.edit import FormMixin

    class AuthorInterestForm(forms.Form):
        message = forms.CharField()

    class AuthorDetail(DetailView, FormMixin):
        model = Author
        form_class = AuthorInterestForm

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

        def get_context_data(self, **kwargs):
            form_class = self.get_form_class()
            form = self.get_form(form_class)
            context = {
                'form': form
            }
            context.update(kwargs)
            return super(AuthorDetail, self).get_context_data(**context)

        def post(self, request, *args, **kwargs):
            form_class = self.get_form_class()
            form = self.get_form(form_class)
            if form.is_valid():
                return self.form_valid(form)
            else:
                return self.form_invalid(form)

        def form_valid(self, form):
            if not self.request.user.is_authenticated():
                return HttpResponseForbidden()
            self.object = self.get_object()
            # record the interest using the message in form.cleaned_data
            return super(AuthorDetail, self).form_valid(form)

:meth:`get_success_url()` is just providing somewhere to redirect to,
which gets used in the default implementation of
:meth:`form_valid()`. We have to provide our own :meth:`post()` as
noted earlier, and override :meth:`get_context_data()` to make the
:class:`Form` available in the context data.

A better solution
-----------------

It should be obvious that the number of subtle interactions between
:class:`FormMixin` and :class:`DetailView` is already testing our
ability to manage things. It's unlikely you'd want to write this kind
of class yourself.

In this case, it would be fairly easy to just write the :meth:`post()`
method yourself, keeping :class:`DetailView` as the only generic
functionality, although writing :class:`Form` handling code involves a
lot of duplication.

Alternatively, it would still be easier than the above approach to
have a separate view for processing the form, which could use
:class:`~django.views.generic.edit.FormView` distinct from
:class:`DetailView` without concerns.

An alternative better solution
------------------------------

What we're really trying to do here is to use two different class
based views from the same URL. So why not do just that? We have a very
clear division here: ``GET`` requests should get the
:class:`DetailView` (with the :class:`Form` added to the context
data), and ``POST`` requests should get the :class:`FormView`. Let's
set up those views first.

The :class:`AuthorDisplay` view is almost the same as :ref:`when we
first introduced AuthorDetail<generic-views-extra-work>`; we have to
write our own :meth:`get_context_data()` to make the
:class:`AuthorInterestForm` available to the template. We'll skip the
:meth:`get_object()` override from before for clarity.

.. code-block:: python

    from django.views.generic import DetailView
    from django import forms
    from books.models import Author

    class AuthorInterestForm(forms.Form):
        message = forms.CharField()

    class AuthorDisplay(DetailView):

        queryset = Author.objects.all()

        def get_context_data(self, **kwargs):
            context = {
                'form': AuthorInterestForm(),
            }
            context.update(kwargs)
            return super(AuthorDisplay, self).get_context_data(**context)

Then the :class:`AuthorInterest` is a simple :class:`FormView`, but we
have to bring in :class:`SingleObjectMixin` so we can find the author
we're talking about, and we have to remember to set
:attr:`template_name` to ensure that form errors will render the same
template as :class:`AuthorDisplay` is using on ``GET``.

.. code-block:: python

    from django.views.generic import FormView
    from django.views.generic.detail import SingleObjectMixin

    class AuthorInterest(FormView, SingleObjectMixin):
        template_name = 'books/author_detail.html'
        form_class = AuthorInterestForm
        model = Author

        def get_context_data(self, **kwargs):
            context = {
                'object': self.get_object(),
            }
            return super(AuthorInterest, self).get_context_data(**context)

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

        def form_valid(self, form):
            if not self.request.user.is_authenticated():
                return HttpResponseForbidden()
            self.object = self.get_object()
            # record the interest using the message in form.cleaned_data
            return super(AuthorInterest, self).form_valid(form)

Finally we bring this together in a new :class:`AuthorDetail` view. We
already know that calling :meth:`as_view()` on a class-based view
gives us something that behaves exactly like a function based view, so
we can do that at the point we choose between the two subviews.

You can of course pass through keyword arguments to :meth:`as_view()`
in the same way you would in your URLconf, such as if you wanted the
:class:`AuthorInterest` behaviour to also appear at another URL but
using a different template.

.. code-block:: python

    from django.views.generic import View

    class AuthorDetail(View):

        def get(self, request, *args, **kwargs):
            view = AuthorDisplay.as_view()
            return view(request, *args, **kwargs)

        def post(self, request, *args, **kwargs):
            view = AuthorInterest.as_view()
            return view(request, *args, **kwargs)

This approach can also be used with any other generic class-based
views or your own class-based views inheriting directly from
:class:`View` or :class:`TemplateView`, as it keeps the different
views as separate as possible.

.. _jsonresponsemixin-example:

More than just HTML
===================

Where class based views shine is when you want to do the same thing many times.
Suppose you're writing an API, and every view should return JSON instead of
rendered HTML.

We can create a mixin class to use in all of our views, handling the
conversion to JSON once.

For example, a simple JSON mixin might look something like this::

    import json
    from django.http import HttpResponse

    class JSONResponseMixin(object):
        """
        A mixin that can be used to render a JSON response.
        """
        response_class = HttpResponse

        def render_to_response(self, context, **response_kwargs):
            """
            Returns a JSON response, transforming 'context' to make the payload.
            """
            response_kwargs['content_type'] = 'application/json'
            return self.response_class(
                self.convert_context_to_json(context),
                **response_kwargs
            )

        def convert_context_to_json(self, context):
            "Convert the context dictionary into a JSON object"
            # Note: This is *EXTREMELY* naive; in reality, you'll need
            # to do much more complex handling to ensure that arbitrary
            # objects -- such as Django model instances or querysets
            # -- can be serialized as JSON.
            return json.dumps(context)

Now we mix this into the base TemplateView::

    from django.views.generic import TemplateView

    class JSONView(JSONResponseMixin, TemplateView):
        pass

Equally we could use our mixin with one of the generic views. We can make our
own version of :class:`~django.views.generic.detail.DetailView` by mixing
:class:`JSONResponseMixin` with the
:class:`~django.views.generic.detail.BaseDetailView` -- (the
:class:`~django.views.generic.detail.DetailView` before template
rendering behavior has been mixed in)::

    class JSONDetailView(JSONResponseMixin, BaseDetailView):
        pass

This view can then be deployed in the same way as any other
:class:`~django.views.generic.detail.DetailView`, with exactly the
same behavior -- except for the format of the response.

If you want to be really adventurous, you could even mix a
:class:`~django.views.generic.detail.DetailView` subclass that is able
to return *both* HTML and JSON content, depending on some property of
the HTTP request, such as a query argument or a HTTP header. Just mix
in both the :class:`JSONResponseMixin` and a
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`,
and override the implementation of :func:`render_to_response()` to defer
to the appropriate subclass depending on the type of response that the user
requested::

    class HybridDetailView(JSONResponseMixin, SingleObjectTemplateResponseMixin, BaseDetailView):
        def render_to_response(self, context):
            # Look for a 'format=json' GET argument
            if self.request.GET.get('format','html') == 'json':
                return JSONResponseMixin.render_to_response(self, context)
            else:
                return SingleObjectTemplateResponseMixin.render_to_response(self, context)

Because of the way that Python resolves method overloading, the local
``render_to_response()`` implementation will override the versions provided by
:class:`JSONResponseMixin` and
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`.