Django2.0手册: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¶

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.

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 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:

{% 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
generic views reference 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¶

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
DetailView generic view provides
the publisher to the context, but how do we get additional information
in that template?

The answer is to subclass 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
get_context_data() for
an example.

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 Making queries
for more information about QuerySet objects,
and see the class-based views reference
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
class-based-views reference 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
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

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):
        # Call the superclass
        object = super().get_object()
        # Record the last accessed date
        object.last_accessed = timezone.now()
        object.save()
        # Return the object
        return object

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
DetailView