Class-based 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_favourite_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.

However, there is; you can subclass DetailView and provide your own implementation of the get_context_data method. The default implementation of this that comes with DetailView simply adds in 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 Publisher, Book

class PublisherDetail(DetailView):

    model = Publisher

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

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 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.

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 books.views import PublisherBookList

urlpatterns = patterns('',
    (r'^books/([\w-]+)/$', 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.args[0])
        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(PublisherBookList, 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)
    name = models.CharField(max_length=200)
    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:

from books.views import AuthorDetailView

urlpatterns = patterns('',
    #...
    url(r'^authors/(?P<pk>\d+)/$', 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.views.generic import DetailView
from django.shortcuts import get_object_or_404
from django.utils import timezone
from books.models import Author

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 = timezone.now()
        object.save()
        # Return the object
        return object