Here again is a recurring theme of this book: at its worst, Web development is boring and monotonous. So far, we’ve covered how 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 display 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
RegisteredUserListViewwould 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 when displaying database data in views. Finally, display views are only one part of Django’s comprehensive class-based view system. For a full introduction and detailed description of the other class-based views Django provides, see Appendix C.
Generic Views of Objects
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:
Now we need to define a view:
Finally hook that view into your urls:
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.
Thus, when (for example) the
APP_DIRS option of a
DjangoTemplates backend is set to True in
TEMPLATES, a template location could be:
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:
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. Appendix C 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.
In Django, if you’re dealing with a model object, this is already done for you. When you are dealing with an object or queryset, Django populates 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.
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:
Providing a useful
context_object_name is always a good idea. Your co-workers 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:
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
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. To pick a simple example, we might want to order a list of books by publication date, with the most recent first:
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:
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.
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?
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:
Next, we’ll write the
PublisherBookList view itself:
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:
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:
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:
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:
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
In this chapter we looked at only a couple of the generic views Django ships with, but the general ideas presented here apply pretty closely to any generic view. Appendix C covers all the available views in detail, and it’s recommended reading if you want to get the most out of this powerful feature.
This concludes the section of this book devoted to advanced usage of models, templates and views. The following chapters cover a range of functions that are very common in modern commercial websites. We will start with a subject essential to building interactive websites – user management.