django1/docs/topics/class-based-views.txt

=========================
Class-based generic views
=========================

.. versionadded:: 1.3

.. note::
    Prior to Django 1.3, generic views were implemented as functions. The
    function-based implementation has been deprecated in favor of the
    class-based approach described here.

    For details on the previous generic views implementation,
    see the :doc:`topic guide </topics/generic-views>` and
    :doc:`detailed reference </ref/generic-views>`.

Writing Web applications can be monotonous, because we repeat certain patterns
again and again. Django tries to take away some of that monotony at the model
and template layers, but Web developers also experience this boredom at the view
level.

Django's *generic views* were developed to ease that pain. They take certain
common idioms and patterns found in view development and abstract them so that
you can quickly write common views of data without having to write too much
code.

We can recognize certain common tasks, like displaying a list of objects, and
write code that displays a list of *any* object. Then the model in question can
be passed as an extra argument to the URLconf.

Django ships with generic views to do the following:

* Perform common "simple" tasks: redirect to a different page and
  render a given template.

* Display list and detail pages for a single object. If we were creating an
  application to manage conferences then a ``TalkListView`` and a
  ``RegisteredUserListView`` would be examples of list views. A single
  talk page is an example of what we call a "detail" view.

* Present date-based objects in year/month/day archive pages,
  associated detail, and "latest" pages.
  `The Django Weblog <https://www.djangoproject.com/weblog/>`_'s
  year, month, and day archives are built with these, as would be a typical
  newspaper's archives.

* Allow users to create, update, and delete objects -- with or
  without authorization.

Taken together, these views provide easy interfaces to perform the most common
tasks developers encounter.


Simple usage
============

Class-based generic views (and any class-based views that inherit from
the base classes Django provides) can be configured in two
ways: subclassing, or passing in arguments directly in the URLconf.

When you subclass a class-based view, you can override attributes
(such as the ``template_name``) or methods (such as ``get_context_data``)
in your subclass to provide new values or methods. Consider, for example,
a view that just displays one template, ``about.html``. Django has a
generic view to do this - :class:`~django.views.generic.base.TemplateView` -
so we can just subclass it, and override the template name::

    # some_app/views.py
    from django.views.generic import TemplateView

    class AboutView(TemplateView):
        template_name = "about.html"

Then, we just need to add this new view into our URLconf. As the class-based
views themselves are classes, we point the URL to the ``as_view`` class method
instead, which is the entry point for class-based views::

    # urls.py
    from django.conf.urls import patterns, url, include
    from some_app.views import AboutView

    urlpatterns = patterns('',
        (r'^about/', AboutView.as_view()),
    )

Alternatively, if you're only changing a few simple attributes on a
class-based view, you can simply pass the new attributes into the ``as_view``
method call itself::

    from django.conf.urls import patterns, url, include
    from django.views.generic import TemplateView

    urlpatterns = patterns('',
        (r'^about/', TemplateView.as_view(template_name="about.html")),
    )

A similar overriding pattern can be used for the ``url`` attribute on
:class:`~django.views.generic.base.RedirectView`, another simple
generic view.


Generic views of objects
========================

:class:`~django.views.generic.base.TemplateView` certainly is useful,
but Django's generic views really shine when it comes to presenting
views of your database content. Because it's such a common task,
Django comes with a handful of built-in generic views that make
generating list and detail views of objects incredibly easy.

Let's take a look at one of these generic views: the "object list" view. We'll
be using these models::

    # models.py
    from django.db import models

    class Publisher(models.Model):
        name = models.CharField(max_length=30)
        address = models.CharField(max_length=50)
        city = models.CharField(max_length=60)
        state_province = models.CharField(max_length=30)
        country = models.CharField(max_length=50)
        website = models.URLField()

        class Meta:
            ordering = ["-name"]

        def __unicode__(self):
            return self.name

    class Book(models.Model):
        title = models.CharField(max_length=100)
        authors = models.ManyToManyField('Author')
        publisher = models.ForeignKey(Publisher)
        publication_date = models.DateField()

To build a list page of all publishers, we'd use a URLconf along these lines::

    from django.conf.urls import patterns, url, include
    from django.views.generic import ListView
    from books.models import Publisher

    urlpatterns = patterns('',
        (r'^publishers/$', ListView.as_view(
            model=Publisher,
        )),
    )

That's all the Python code we need to write. We still need to write a template,
however. We could explicitly tell the view which template to use
by including a ``template_name`` key in the arguments to as_view, but in
the absence of an explicit template Django will infer one from the object's
name. In this case, the inferred template will be
``"books/publisher_list.html"`` -- the "books" part comes from the name of the
app that defines the model, while the "publisher" bit is just the lowercased
version of the model's name.

.. note::
    Thus, when (for example) the :class:`django.template.loaders.app_directories.Loader`
    template loader is enabled in :setting:`TEMPLATE_LOADERS`, the template
    location would be::

        /path/to/project/books/templates/books/publisher_list.html

.. highlightlang:: html+django

This template will be rendered against a context containing a variable called
``object_list`` that contains all the publisher objects. A very simple template
might look like the following::

    {% extends "base.html" %}

    {% block content %}
        <h2>Publishers</h2>
        <ul>
            {% for publisher in object_list %}
                <li>{{ publisher.name }}</li>
            {% endfor %}
        </ul>
    {% endblock %}

That's really all there is to it. All the cool features of generic views come
from changing the "info" dictionary passed to the generic view. The
:doc:`generic views reference</ref/class-based-views>` documents all the generic
views and their options in detail; the rest of this document will consider
some of the common ways you might customize and extend generic views.


Extending generic views
=======================

.. highlightlang:: python

There's no question that using generic views can speed up development
substantially. In most projects, however, there comes a moment when the
generic views no longer suffice. Indeed, the most common question asked by new
Django developers is how to make generic views handle a wider array of
situations.

This is one of the reasons generic views were redesigned for the 1.3 release -
previously, they were just view functions with a bewildering array of options;
now, rather than passing in a large amount of configuration in the URLconf,
the recommended way to extend generic views is to subclass them, and override
their attributes or methods.


Making "friendly" template contexts
-----------------------------------

You might have noticed that our sample publisher list template stores
all the publishers in a variable named ``object_list``. While this
works just fine, it isn't all that "friendly" to template authors:
they have to "just know" that they're dealing with publishers here.

Well, if you're dealing with a model object, this is already done for
you. When you are dealing with an object or queryset, Django is able
to populate the context using the verbose name (or the plural verbose
name, in the case of a list of objects) of the object being displayed.
This is provided in addition to the default ``object_list`` entry, but
contains exactly the same data.

If the verbose name (or plural verbose name) still isn't a good match,
you can manually set the name of the context variable. The
``context_object_name`` attribute on a generic view specifies the
context variable to use. In this example, we'll override it in the
URLconf, since it's a simple change:

.. parsed-literal::

    urlpatterns = patterns('',
        (r'^publishers/$', ListView.as_view(
            model=Publisher,
            **context_object_name="publisher_list",**
        )),
    )

Providing a useful ``context_object_name`` is always a good idea. Your
coworkers who design templates will thank you.


Adding extra context
--------------------

Often you simply need to present some extra information beyond that
provided by the generic view. For example, think of showing a list of
all the books on each publisher detail page. The
:class:`~django.views.generic.detail.DetailView` generic view provides
the publisher to the context, but it seems there's no way to get
additional information in that template.

However, there is; you can subclass
:class:`~django.views.generic.detail.DetailView` and provide your own
implementation of the ``get_context_data`` method. The default
implementation of this that comes with
:class:`~django.views.generic.detail.DetailView` simply adds in the
object being displayed to the template, but you can override it to show
more::

    from django.views.generic import DetailView
    from books.models import Publisher, Book

    class PublisherDetailView(DetailView):

        context_object_name = "publisher"
        model = Publisher

        def get_context_data(self, **kwargs):
            # Call the base implementation first to get a context
            context = super(PublisherDetailView, self).get_context_data(**kwargs)
            # Add in a QuerySet of all the books
            context['book_list'] = Book.objects.all()
            return context

.. note::

    Generally, get_context_data will merge the context data of all parent classes
    with those of the current class.  To preserve this behavior in your own classes
    where you want to alter the context, you should be sure to call
    get_context_data on the super class. When no two classes try to define the same
    key, this will give the expected results. However if any class attempts to
    override a key after parent classes have set it (after the call to super), any
    children of that class will also need to explictly set it after super if they
    want to be sure to override all parents.

Viewing subsets of objects
--------------------------

Now let's take a closer look at the ``model`` argument we've been
using all along. The ``model`` argument, which specifies the database
model that the view will operate upon, is available on all the
generic views that operate on a single object or a collection of
objects. However, the ``model`` argument is not the only way to
specify the objects that the view will operate upon -- you can also
specify the list of objects using the ``queryset`` argument::

    from django.views.generic import DetailView
    from books.models import Publisher, Book

    class PublisherDetailView(DetailView):

        context_object_name = "publisher"
        queryset = Publisher.objects.all()

Specifying ``model = Publisher`` is really just shorthand for saying
``queryset = Publisher.objects.all()``. However, by using ``queryset``
to define a filtered list of objects you can be more specific about the
objects that will be visible in the view (see :doc:`/topics/db/queries`
for more information about :class:`QuerySet` objects, and see the
:doc:`class-based views reference </ref/class-based-views>` for the complete
details).

To pick a simple example, we might want to order a list of books by
publication date, with the most recent first::

    urlpatterns = patterns('',
        (r'^publishers/$', ListView.as_view(
            queryset=Publisher.objects.all(),
            context_object_name="publisher_list",
        )),
        (r'^books/$', ListView.as_view(
            queryset=Book.objects.order_by("-publication_date"),
            context_object_name="book_list",
        )),
    )


That's a pretty simple example, but it illustrates the idea nicely. Of course,
you'll usually want to do more than just reorder objects. If you want to
present a list of books by a particular publisher, you can use the same
technique (here, illustrated using subclassing rather than by passing arguments
in the URLconf)::

    from django.views.generic import ListView
    from books.models import Book

    class AcmeBookListView(ListView):

        context_object_name = "book_list"
        queryset = Book.objects.filter(publisher__name="Acme Publishing")
        template_name = "books/acme_list.html"

Notice that along with a filtered ``queryset``, we're also using a custom
template name. If we didn't, the generic view would use the same template as the
"vanilla" object list, which might not be what we want.

Also notice that this isn't a very elegant way of doing publisher-specific
books. If we want to add another publisher page, we'd need another handful of
lines in the URLconf, and more than a few publishers would get unreasonable.
We'll deal with this problem in the next section.

.. note::

    If you get a 404 when requesting ``/books/acme/``, check to ensure you
    actually have a Publisher with the name 'ACME Publishing'.  Generic
    views have an ``allow_empty`` parameter for this case.  See the
    :doc:`class-based-views reference</ref/class-based-views>` for more details.


Dynamic filtering
-----------------

Another common need is to filter down the objects given in a list page by some
key in the URL. Earlier we hard-coded the publisher's name in the URLconf, but
what if we wanted to write a view that displayed all the books by some arbitrary
publisher?

Handily, the ``ListView`` has a
:meth:`~django.views.generic.detail.ListView.get_queryset` method we can
override. Previously, it has just been returning the value of the ``queryset``
attribute, but now we can add more logic.

The key part to making this work is that when class-based views are called,
various useful things are stored on ``self``; as well as the request
(``self.request``) this includes the positional (``self.args``) and name-based
(``self.kwargs``) arguments captured according to the URLconf.

Here, we have a URLconf with a single captured group::

    from books.views import PublisherBookListView

    urlpatterns = patterns('',
        (r'^books/(\w+)/$', PublisherBookListView.as_view()),
    )

Next, we'll write the ``PublisherBookListView`` view itself::

    from django.shortcuts import get_object_or_404
    from django.views.generic import ListView
    from books.models import Book, Publisher

    class PublisherBookListView(ListView):

        context_object_name = "book_list"
        template_name = "books/books_by_publisher.html"

        def get_queryset(self):
            publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
            return Book.objects.filter(publisher=publisher)

As you can see, it's quite easy to add more logic to the queryset selection;
if we wanted, we could use ``self.request.user`` to filter using the current
user, or other more complex logic.

We can also add the publisher into the context at the same time, so we can
use it in the template::

    class PublisherBookListView(ListView):

        context_object_name = "book_list"
        template_name = "books/books_by_publisher.html"

        def get_queryset(self):
            self.publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
            return Book.objects.filter(publisher=self.publisher)

        def get_context_data(self, **kwargs):
            # Call the base implementation first to get a context
            context = super(PublisherBookListView, self).get_context_data(**kwargs)
            # Add in the publisher
            context['publisher'] = self.publisher
            return context

Performing extra work
---------------------

The last common pattern we'll look at involves doing some extra work before
or after calling the generic view.

Imagine we had a ``last_accessed`` field on our ``Author`` object that we were
using to keep track of the last time anybody looked at that author::

    # models.py

    class Author(models.Model):
        salutation = models.CharField(max_length=10)
        first_name = models.CharField(max_length=30)
        last_name = models.CharField(max_length=40)
        email = models.EmailField()
        headshot = models.ImageField(upload_to='/tmp')
        last_accessed = models.DateTimeField()

The generic ``DetailView`` class, of course, wouldn't know anything about this
field, but once again we could easily write a custom view to keep that field
updated.

First, we'd need to add an author detail bit in the URLconf to point to a
custom view:

.. parsed-literal::

    from books.views import AuthorDetailView

    urlpatterns = patterns('',
        #...
        **(r'^authors/(?P<pk>\\d+)/$', AuthorDetailView.as_view()),**
    )

Then we'd write our new view -- ``get_object`` is the method that retrieves the
object -- so we simply override it and wrap the call::

    import datetime
    from books.models import Author
    from django.views.generic import DetailView
    from django.shortcuts import get_object_or_404

    class AuthorDetailView(DetailView):

        queryset = Author.objects.all()

        def get_object(self):
            # Call the superclass
            object = super(AuthorDetailView, self).get_object()
            # Record the last accessed date
            object.last_accessed = datetime.datetime.now()
            object.save()
            # Return the object
            return object

.. note::

    This code won't actually work unless you create a
    ``books/author_detail.html`` template.

.. note::

    The URLconf here uses the named group ``pk`` - this name is the default
    name that ``DetailView`` uses to find the value of the primary key used to
    filter the queryset.

    If you want to change it, you'll need to do your own ``get()`` call
    on ``self.queryset`` using the new named parameter from ``self.kwargs``.

More than just HTML
-------------------

So far, we've been focusing on rendering templates to generate
responses. However, that's not all generic views can do.

Each generic view is composed out of a series of mixins, and each
mixin contributes a little piece of the entire view. Some of these
mixins -- such as
:class:`~django.views.generic.base.TemplateResponseMixin` -- are
specifically designed for rendering content to an HTML response using a
template. However, you can write your own mixins that perform
different rendering behavior.

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

    from django import http
    from django.utils import simplejson as json

    class JSONResponseMixin(object):
        def render_to_response(self, context):
            "Returns a JSON response containing 'context' as payload"
            return self.get_json_response(self.convert_context_to_json(context))

        def get_json_response(self, content, **httpresponse_kwargs):
            "Construct an `HttpResponse` object."
            return http.HttpResponse(content,
                                     content_type='application/json',
                                     **httpresponse_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)

Then, you could build a JSON-returning
:class:`~django.views.generic.detail.DetailView` by mixing your
: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`.

Decorating class-based views
============================

.. highlightlang:: python

The extension of class-based views isn't limited to using mixins. You
can use also use decorators.

Decorating in URLconf
---------------------

The simplest way of decorating class-based views is to decorate the
result of the :meth:`~django.views.generic.base.View.as_view` method.
The easiest place to do this is in the URLconf where you deploy your
view::

    from django.contrib.auth.decorators import login_required, permission_required
    from django.views.generic import TemplateView

    from .views import VoteView

    urlpatterns = patterns('',
        (r'^about/', login_required(TemplateView.as_view(template_name="secret.html"))),
        (r'^vote/', permission_required('polls.can_vote')(VoteView.as_view())),
    )

This approach applies the decorator on a per-instance basis. If you
want every instance of a view to be decorated, you need to take a
different approach.

.. _decorating-class-based-views:

Decorating the class
--------------------

To decorate every instance of a class-based view, you need to decorate
the class definition itself. To do this you apply the decorator to the
:meth:`~django.views.generic.base.View.dispatch` method of the class.

A method on a class isn't quite the same as a standalone function, so
you can't just apply a function decorator to the method -- you need to
transform it into a method decorator first. The ``method_decorator``
decorator transforms a function decorator into a method decorator so
that it can be used on an instance method. For example::

    from django.contrib.auth.decorators import login_required
    from django.utils.decorators import method_decorator
    from django.views.generic import TemplateView

    class ProtectedView(TemplateView):
        template_name = 'secret.html'

        @method_decorator(login_required)
        def dispatch(self, *args, **kwargs):
            return super(ProtectedView, self).dispatch(*args, **kwargs)

In this example, every instance of ``ProtectedView`` will have
login protection.

.. note::

    ``method_decorator`` passes ``*args`` and ``**kwargs``
    as parameters to the decorated method on the class. If your method
    does not accept a compatible set of parameters it will raise a
    ``TypeError`` exception.