==================================

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 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 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 to help generate list and detail views of objects.

Let's start by looking at some examples of showing a list of objects or an

individual object.

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 PublisherListView(ListView):

        model = Publisher

Finally hook that view into your urls::

    # urls.py

    from django.urls import path

    from books.views import PublisherListView

    urlpatterns = [

        path('publishers/', PublisherListView.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 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 template might look

like this:

.. 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 lowercased 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 PublisherListView(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 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 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 PublisherDetailView(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 PublisherDetailView(DetailView):

        context_object_name = 'publisher'

        queryset = Publisher.objects.all()

Specifying ``model = Publisher`` is 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 an 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 BookListView(ListView):

        queryset = Book.objects.order_by('-publication_date')

        context_object_name = 'book_list'

That's a pretty minimal example, but it illustrates the idea nicely. 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 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/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. By default, it returns the value of the ``queryset`` attribute,

but we can use it to 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 PublisherBookListView

    urlpatterns = [

        path('books/<publisher>/', PublisherBookListView.as_view()),

    ]

Next, we'll write the ``PublisherBookListView`` 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 PublisherBookListView(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)

Using ``get_queryset`` to add logic to the queryset selection is as convenient

as it is powerful. For instance, 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 wouldn't know anything about this field, but

once again we could 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 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

    :attr:`~django.views.generic.detail.SingleObjectMixin.pk_url_kwarg`

    on the view.