==================================
Built-in class-based 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:

* 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.

* 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.


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

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.

That said, generic views will have a limit. If you find you're struggling to
implement your view as a subclass of a generic view, then you may find it more
effective to write just the code you need, using your own class-based or
functional views.

More examples of generic views are available in some third party applications,
or you could write your own as needed.


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 start by looking at some examples of showing a list of objects or an
individual object.

.. comment: link here to the other topic pages (form handling, date based, mixins)

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 __str__(self):
            return self.name

    class Author(models.Model):
        salutation = models.CharField(max_length=10)
        name = models.CharField(max_length=200)
        email = models.EmailField()
        headshot = models.ImageField(upload_to='author_headshots')

        def __str__(self):
            return self.name

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

Now we need to define a view::

    # views.py
    from django.views.generic import ListView
    from books.models import Publisher

    class PublisherList(ListView):
        model = Publisher

Finally hook that view into your urls::

    # urls.py
    from django.urls import path
    from books.views import PublisherList

    urlpatterns = [
        path('publishers/', PublisherList.as_view()),
    ]

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 adding a
``template_name`` attribute to the 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 ``APP_DIRS`` option of a ``DjangoTemplates``
    backend is set to True in :setting:`TEMPLATES`, a template location could
    be: /path/to/project/books/templates/books/publisher_list.html

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:

.. code-block:: html+django

    {% 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 attributes set on the generic view. The
:doc:`generic views reference</ref/class-based-views/index>` 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.


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 lower cased version of the model class' name. This is
provided in addition to the default ``object_list`` entry, but contains exactly
the same data, i.e. ``publisher_list``.

If this 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::

    # views.py
    from django.views.generic import ListView
    from books.models import Publisher

    class PublisherList(ListView):
        model = Publisher
        context_object_name = 'my_favorite_publishers'

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


.. _adding-extra-context:

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 how do we get additional information
in that template?

The answer is to subclass :class:`~django.views.generic.detail.DetailView`
and provide your own implementation of the ``get_context_data`` method.
The default implementation simply adds the object being displayed to the
template, but you can override it to send more::

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

    class PublisherDetail(DetailView):

        model = Publisher

        def get_context_data(self, **kwargs):
            # Call the base implementation first to get a context
            context = super().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 explicitly set it
    after super if they want to be sure to override all parents. If you're
    having trouble, review the method resolution order of your view.

    Another consideration is that the context data from class-based generic
    views will override data provided by context processors; see
    :meth:`~django.views.generic.detail.SingleObjectMixin.get_context_data` for
    an example.

.. _generic-views-list-subsets:

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

    class PublisherDetail(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:`~django.db.models.query.QuerySet` objects,
and see the :doc:`class-based views reference </ref/class-based-views/index>`
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::

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

    class BookList(ListView):
        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::

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

    class AcmeBookList(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/index>` 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.list.MultipleObjectMixin.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::

    # urls.py
    from django.urls import path
    from books.views import PublisherBookList

    urlpatterns = [
        path('books/<publisher>/', PublisherBookList.as_view()),
    ]

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

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

    class PublisherBookList(ListView):

        template_name = 'books/books_by_publisher.html'

        def get_queryset(self):
            self.publisher = get_object_or_404(Publisher, name=self.kwargs['publisher'])
            return Book.objects.filter(publisher=self.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::

        # ...

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

.. _generic-views-extra-work:

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`` model that we were
using to keep track of the last time anybody looked at that author::

    # models.py
    from django.db import models

    class Author(models.Model):
        salutation = models.CharField(max_length=10)
        name = models.CharField(max_length=200)
        email = models.EmailField()
        headshot = models.ImageField(upload_to='author_headshots')
        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::

    from django.urls import path
    from books.views import AuthorDetailView

    urlpatterns = [
        #...
        path('authors/<int:pk>/', AuthorDetailView.as_view(), name='author-detail'),
    ]

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::

    from django.utils import timezone
    from django.views.generic import DetailView
    from books.models import Author

    class AuthorDetailView(DetailView):

        queryset = Author.objects.all()

        def get_object(self):
            obj = super().get_object()
            # Record the last accessed date
            obj.last_accessed = timezone.now()
            obj.save()
            return obj

.. 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 call the group something else, you can set ``pk_url_kwarg``
    on the view. More details can be found in the reference for
    :class:`~django.views.generic.detail.DetailView`
