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627 lines
24 KiB
Text
627 lines
24 KiB
Text
=========================
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Class-based generic views
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=========================
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.. versionadded:: 1.3
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.. note::
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Prior to Django 1.3, generic views were implemented as functions. The
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function-based implementation has been removed in favor of the
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class-based approach described here.
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Writing Web applications can be monotonous, because we repeat certain patterns
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again and again. Django tries to take away some of that monotony at the model
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and template layers, but Web developers also experience this boredom at the view
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level.
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Django's *generic views* were developed to ease that pain. They take certain
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common idioms and patterns found in view development and abstract them so that
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you can quickly write common views of data without having to write too much
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code.
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We can recognize certain common tasks, like displaying a list of objects, and
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write code that displays a list of *any* object. Then the model in question can
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be passed as an extra argument to the URLconf.
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Django ships with generic views to do the following:
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* Perform common "simple" tasks: redirect to a different page and
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render a given template.
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* Display list and detail pages for a single object. If we were creating an
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application to manage conferences then a ``TalkListView`` and a
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``RegisteredUserListView`` would be examples of list views. A single
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talk page is an example of what we call a "detail" view.
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* Present date-based objects in year/month/day archive pages,
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associated detail, and "latest" pages.
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`The Django Weblog <https://www.djangoproject.com/weblog/>`_'s
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year, month, and day archives are built with these, as would be a typical
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newspaper's archives.
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* Allow users to create, update, and delete objects -- with or
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without authorization.
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Taken together, these views provide easy interfaces to perform the most common
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tasks developers encounter.
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Simple usage
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============
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Class-based generic views (and any class-based views that inherit from
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the base classes Django provides) can be configured in two
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ways: subclassing, or passing in arguments directly in the URLconf.
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When you subclass a class-based view, you can override attributes
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(such as the ``template_name``) or methods (such as ``get_context_data``)
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in your subclass to provide new values or methods. Consider, for example,
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a view that just displays one template, ``about.html``. Django has a
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generic view to do this - :class:`~django.views.generic.base.TemplateView` -
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so we can just subclass it, and override the template name::
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# some_app/views.py
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from django.views.generic import TemplateView
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class AboutView(TemplateView):
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template_name = "about.html"
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Then, we just need to add this new view into our URLconf. As the class-based
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views themselves are classes, we point the URL to the ``as_view`` class method
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instead, which is the entry point for class-based views::
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# urls.py
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from django.conf.urls import patterns, url, include
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from some_app.views import AboutView
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urlpatterns = patterns('',
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(r'^about/', AboutView.as_view()),
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)
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Alternatively, if you're only changing a few simple attributes on a
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class-based view, you can simply pass the new attributes into the ``as_view``
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method call itself::
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from django.conf.urls import patterns, url, include
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from django.views.generic import TemplateView
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urlpatterns = patterns('',
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(r'^about/', TemplateView.as_view(template_name="about.html")),
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)
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A similar overriding pattern can be used for the ``url`` attribute on
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:class:`~django.views.generic.base.RedirectView`, another simple
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generic view.
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Generic views of objects
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========================
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:class:`~django.views.generic.base.TemplateView` certainly is useful,
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but Django's generic views really shine when it comes to presenting
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views of your database content. Because it's such a common task,
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Django comes with a handful of built-in generic views that make
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generating list and detail views of objects incredibly easy.
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Let's take a look at one of these generic views: the "object list" view. We'll
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be using these models::
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# models.py
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from django.db import models
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class Publisher(models.Model):
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name = models.CharField(max_length=30)
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address = models.CharField(max_length=50)
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city = models.CharField(max_length=60)
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state_province = models.CharField(max_length=30)
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country = models.CharField(max_length=50)
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website = models.URLField()
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class Meta:
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ordering = ["-name"]
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def __unicode__(self):
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return self.name
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class Book(models.Model):
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title = models.CharField(max_length=100)
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authors = models.ManyToManyField('Author')
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publisher = models.ForeignKey(Publisher)
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publication_date = models.DateField()
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To build a list page of all publishers, we'd use a URLconf along these lines::
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from django.conf.urls import patterns, url, include
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from django.views.generic import ListView
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from books.models import Publisher
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urlpatterns = patterns('',
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(r'^publishers/$', ListView.as_view(
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model=Publisher,
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)),
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)
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That's all the Python code we need to write. We still need to write a template,
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however. We could explicitly tell the view which template to use
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by including a ``template_name`` key in the arguments to as_view, but in
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the absence of an explicit template Django will infer one from the object's
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name. In this case, the inferred template will be
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``"books/publisher_list.html"`` -- the "books" part comes from the name of the
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app that defines the model, while the "publisher" bit is just the lowercased
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version of the model's name.
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.. note::
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Thus, when (for example) the :class:`django.template.loaders.app_directories.Loader`
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template loader is enabled in :setting:`TEMPLATE_LOADERS`, the template
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location would be::
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/path/to/project/books/templates/books/publisher_list.html
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.. highlightlang:: html+django
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This template will be rendered against a context containing a variable called
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``object_list`` that contains all the publisher objects. A very simple template
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might look like the following::
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{% extends "base.html" %}
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{% block content %}
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<h2>Publishers</h2>
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<ul>
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{% for publisher in object_list %}
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<li>{{ publisher.name }}</li>
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{% endfor %}
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</ul>
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{% endblock %}
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That's really all there is to it. All the cool features of generic views come
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from changing the "info" dictionary passed to the generic view. The
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:doc:`generic views reference</ref/class-based-views>` documents all the generic
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views and their options in detail; the rest of this document will consider
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some of the common ways you might customize and extend generic views.
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Extending generic views
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=======================
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.. highlightlang:: python
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There's no question that using generic views can speed up development
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substantially. In most projects, however, there comes a moment when the
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generic views no longer suffice. Indeed, the most common question asked by new
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Django developers is how to make generic views handle a wider array of
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situations.
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This is one of the reasons generic views were redesigned for the 1.3 release -
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previously, they were just view functions with a bewildering array of options;
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now, rather than passing in a large amount of configuration in the URLconf,
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the recommended way to extend generic views is to subclass them, and override
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their attributes or methods.
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Making "friendly" template contexts
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-----------------------------------
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You might have noticed that our sample publisher list template stores
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all the publishers in a variable named ``object_list``. While this
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works just fine, it isn't all that "friendly" to template authors:
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they have to "just know" that they're dealing with publishers here.
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Well, if you're dealing with a model object, this is already done for
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you. When you are dealing with an object or queryset, Django is able
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to populate the context using the verbose name (or the plural verbose
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name, in the case of a list of objects) of the object being displayed.
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This is provided in addition to the default ``object_list`` entry, but
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contains exactly the same data.
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If the verbose name (or plural verbose name) still isn't a good match,
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you can manually set the name of the context variable. The
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``context_object_name`` attribute on a generic view specifies the
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context variable to use. In this example, we'll override it in the
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URLconf, since it's a simple change:
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.. parsed-literal::
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urlpatterns = patterns('',
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(r'^publishers/$', ListView.as_view(
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model=Publisher,
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**context_object_name="publisher_list",**
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)),
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)
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Providing a useful ``context_object_name`` is always a good idea. Your
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coworkers who design templates will thank you.
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Adding extra context
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--------------------
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Often you simply need to present some extra information beyond that
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provided by the generic view. For example, think of showing a list of
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all the books on each publisher detail page. The
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:class:`~django.views.generic.detail.DetailView` generic view provides
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the publisher to the context, but it seems there's no way to get
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additional information in that template.
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However, there is; you can subclass
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:class:`~django.views.generic.detail.DetailView` and provide your own
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implementation of the ``get_context_data`` method. The default
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implementation of this that comes with
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:class:`~django.views.generic.detail.DetailView` simply adds in the
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object being displayed to the template, but you can override it to show
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more::
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from django.views.generic import DetailView
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from books.models import Publisher, Book
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class PublisherDetailView(DetailView):
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context_object_name = "publisher"
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model = Publisher
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def get_context_data(self, **kwargs):
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# Call the base implementation first to get a context
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context = super(PublisherDetailView, self).get_context_data(**kwargs)
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# Add in a QuerySet of all the books
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context['book_list'] = Book.objects.all()
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return context
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.. note::
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Generally, get_context_data will merge the context data of all parent classes
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with those of the current class. To preserve this behavior in your own classes
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where you want to alter the context, you should be sure to call
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get_context_data on the super class. When no two classes try to define the same
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key, this will give the expected results. However if any class attempts to
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override a key after parent classes have set it (after the call to super), any
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children of that class will also need to explictly set it after super if they
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want to be sure to override all parents.
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Viewing subsets of objects
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--------------------------
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Now let's take a closer look at the ``model`` argument we've been
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using all along. The ``model`` argument, which specifies the database
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model that the view will operate upon, is available on all the
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generic views that operate on a single object or a collection of
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objects. However, the ``model`` argument is not the only way to
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specify the objects that the view will operate upon -- you can also
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specify the list of objects using the ``queryset`` argument::
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from django.views.generic import DetailView
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from books.models import Publisher, Book
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class PublisherDetailView(DetailView):
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context_object_name = "publisher"
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queryset = Publisher.objects.all()
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Specifying ``model = Publisher`` is really just shorthand for saying
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``queryset = Publisher.objects.all()``. However, by using ``queryset``
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to define a filtered list of objects you can be more specific about the
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objects that will be visible in the view (see :doc:`/topics/db/queries`
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for more information about :class:`QuerySet` objects, and see the
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:doc:`class-based views reference </ref/class-based-views>` for the complete
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details).
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To pick a simple example, we might want to order a list of books by
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publication date, with the most recent first::
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urlpatterns = patterns('',
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(r'^publishers/$', ListView.as_view(
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queryset=Publisher.objects.all(),
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context_object_name="publisher_list",
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)),
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(r'^books/$', ListView.as_view(
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queryset=Book.objects.order_by("-publication_date"),
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context_object_name="book_list",
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)),
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)
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That's a pretty simple example, but it illustrates the idea nicely. Of course,
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you'll usually want to do more than just reorder objects. If you want to
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present a list of books by a particular publisher, you can use the same
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technique (here, illustrated using subclassing rather than by passing arguments
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in the URLconf)::
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from django.views.generic import ListView
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from books.models import Book
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class AcmeBookListView(ListView):
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context_object_name = "book_list"
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queryset = Book.objects.filter(publisher__name="Acme Publishing")
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template_name = "books/acme_list.html"
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Notice that along with a filtered ``queryset``, we're also using a custom
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template name. If we didn't, the generic view would use the same template as the
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"vanilla" object list, which might not be what we want.
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Also notice that this isn't a very elegant way of doing publisher-specific
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books. If we want to add another publisher page, we'd need another handful of
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lines in the URLconf, and more than a few publishers would get unreasonable.
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We'll deal with this problem in the next section.
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.. note::
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If you get a 404 when requesting ``/books/acme/``, check to ensure you
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actually have a Publisher with the name 'ACME Publishing'. Generic
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views have an ``allow_empty`` parameter for this case. See the
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:doc:`class-based-views reference</ref/class-based-views>` for more details.
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Dynamic filtering
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-----------------
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Another common need is to filter down the objects given in a list page by some
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key in the URL. Earlier we hard-coded the publisher's name in the URLconf, but
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what if we wanted to write a view that displayed all the books by some arbitrary
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publisher?
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Handily, the ``ListView`` has a
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:meth:`~django.views.generic.detail.ListView.get_queryset` method we can
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override. Previously, it has just been returning the value of the ``queryset``
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attribute, but now we can add more logic.
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The key part to making this work is that when class-based views are called,
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various useful things are stored on ``self``; as well as the request
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(``self.request``) this includes the positional (``self.args``) and name-based
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(``self.kwargs``) arguments captured according to the URLconf.
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Here, we have a URLconf with a single captured group::
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from books.views import PublisherBookListView
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urlpatterns = patterns('',
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(r'^books/(\w+)/$', PublisherBookListView.as_view()),
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)
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Next, we'll write the ``PublisherBookListView`` view itself::
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from django.shortcuts import get_object_or_404
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from django.views.generic import ListView
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from books.models import Book, Publisher
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class PublisherBookListView(ListView):
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context_object_name = "book_list"
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template_name = "books/books_by_publisher.html"
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def get_queryset(self):
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publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
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return Book.objects.filter(publisher=publisher)
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As you can see, it's quite easy to add more logic to the queryset selection;
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if we wanted, we could use ``self.request.user`` to filter using the current
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user, or other more complex logic.
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We can also add the publisher into the context at the same time, so we can
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use it in the template::
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class PublisherBookListView(ListView):
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context_object_name = "book_list"
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template_name = "books/books_by_publisher.html"
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def get_queryset(self):
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self.publisher = get_object_or_404(Publisher, name__iexact=self.args[0])
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return Book.objects.filter(publisher=self.publisher)
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def get_context_data(self, **kwargs):
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# Call the base implementation first to get a context
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context = super(PublisherBookListView, self).get_context_data(**kwargs)
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# Add in the publisher
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context['publisher'] = self.publisher
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return context
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Performing extra work
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---------------------
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The last common pattern we'll look at involves doing some extra work before
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or after calling the generic view.
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Imagine we had a ``last_accessed`` field on our ``Author`` object that we were
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using to keep track of the last time anybody looked at that author::
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# models.py
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class Author(models.Model):
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salutation = models.CharField(max_length=10)
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first_name = models.CharField(max_length=30)
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last_name = models.CharField(max_length=40)
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email = models.EmailField()
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headshot = models.ImageField(upload_to='/tmp')
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last_accessed = models.DateTimeField()
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The generic ``DetailView`` class, of course, wouldn't know anything about this
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field, but once again we could easily write a custom view to keep that field
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updated.
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First, we'd need to add an author detail bit in the URLconf to point to a
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custom view:
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.. parsed-literal::
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from books.views import AuthorDetailView
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urlpatterns = patterns('',
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#...
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**(r'^authors/(?P<pk>\\d+)/$', AuthorDetailView.as_view()),**
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)
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Then we'd write our new view -- ``get_object`` is the method that retrieves the
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object -- so we simply override it and wrap the call::
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import datetime
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from books.models import Author
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from django.views.generic import DetailView
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from django.shortcuts import get_object_or_404
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class AuthorDetailView(DetailView):
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queryset = Author.objects.all()
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def get_object(self):
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# Call the superclass
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object = super(AuthorDetailView, self).get_object()
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# Record the last accessed date
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object.last_accessed = datetime.datetime.now()
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object.save()
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# Return the object
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return object
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.. note::
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This code won't actually work unless you create a
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``books/author_detail.html`` template.
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.. note::
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The URLconf here uses the named group ``pk`` - this name is the default
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name that ``DetailView`` uses to find the value of the primary key used to
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filter the queryset.
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If you want to change it, you'll need to do your own ``get()`` call
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on ``self.queryset`` using the new named parameter from ``self.kwargs``.
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More than just HTML
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-------------------
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So far, we've been focusing on rendering templates to generate
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responses. However, that's not all generic views can do.
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Each generic view is composed out of a series of mixins, and each
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mixin contributes a little piece of the entire view. Some of these
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mixins -- such as
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:class:`~django.views.generic.base.TemplateResponseMixin` -- are
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specifically designed for rendering content to an HTML response using a
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template. However, you can write your own mixins that perform
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different rendering behavior.
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For example, a simple JSON mixin might look something like this::
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import json
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from django import http
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class JSONResponseMixin(object):
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def render_to_response(self, context):
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"Returns a JSON response containing 'context' as payload"
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return self.get_json_response(self.convert_context_to_json(context))
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def get_json_response(self, content, **httpresponse_kwargs):
|
|
"Construct an `HttpResponse` object."
|
|
return http.HttpResponse(content,
|
|
content_type='application/json',
|
|
**httpresponse_kwargs)
|
|
|
|
def convert_context_to_json(self, context):
|
|
"Convert the context dictionary into a JSON object"
|
|
# Note: This is *EXTREMELY* naive; in reality, you'll need
|
|
# to do much more complex handling to ensure that arbitrary
|
|
# objects -- such as Django model instances or querysets
|
|
# -- can be serialized as JSON.
|
|
return json.dumps(context)
|
|
|
|
Then, you could build a JSON-returning
|
|
:class:`~django.views.generic.detail.DetailView` by mixing your
|
|
:class:`JSONResponseMixin` with the
|
|
:class:`~django.views.generic.detail.BaseDetailView` -- (the
|
|
:class:`~django.views.generic.detail.DetailView` before template
|
|
rendering behavior has been mixed in)::
|
|
|
|
class JSONDetailView(JSONResponseMixin, BaseDetailView):
|
|
pass
|
|
|
|
This view can then be deployed in the same way as any other
|
|
:class:`~django.views.generic.detail.DetailView`, with exactly the
|
|
same behavior -- except for the format of the response.
|
|
|
|
If you want to be really adventurous, you could even mix a
|
|
:class:`~django.views.generic.detail.DetailView` subclass that is able
|
|
to return *both* HTML and JSON content, depending on some property of
|
|
the HTTP request, such as a query argument or a HTTP header. Just mix
|
|
in both the :class:`JSONResponseMixin` and a
|
|
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`,
|
|
and override the implementation of :func:`render_to_response()` to defer
|
|
to the appropriate subclass depending on the type of response that the user
|
|
requested::
|
|
|
|
class HybridDetailView(JSONResponseMixin, SingleObjectTemplateResponseMixin, BaseDetailView):
|
|
def render_to_response(self, context):
|
|
# Look for a 'format=json' GET argument
|
|
if self.request.GET.get('format','html') == 'json':
|
|
return JSONResponseMixin.render_to_response(self, context)
|
|
else:
|
|
return SingleObjectTemplateResponseMixin.render_to_response(self, context)
|
|
|
|
Because of the way that Python resolves method overloading, the local
|
|
``render_to_response()`` implementation will override the versions provided by
|
|
:class:`JSONResponseMixin` and
|
|
:class:`~django.views.generic.detail.SingleObjectTemplateResponseMixin`.
|
|
|
|
Decorating class-based views
|
|
============================
|
|
|
|
.. highlightlang:: python
|
|
|
|
The extension of class-based views isn't limited to using mixins. You
|
|
can use also use decorators.
|
|
|
|
Decorating in URLconf
|
|
---------------------
|
|
|
|
The simplest way of decorating class-based views is to decorate the
|
|
result of the :meth:`~django.views.generic.base.View.as_view` method.
|
|
The easiest place to do this is in the URLconf where you deploy your
|
|
view::
|
|
|
|
from django.contrib.auth.decorators import login_required, permission_required
|
|
from django.views.generic import TemplateView
|
|
|
|
from .views import VoteView
|
|
|
|
urlpatterns = patterns('',
|
|
(r'^about/', login_required(TemplateView.as_view(template_name="secret.html"))),
|
|
(r'^vote/', permission_required('polls.can_vote')(VoteView.as_view())),
|
|
)
|
|
|
|
This approach applies the decorator on a per-instance basis. If you
|
|
want every instance of a view to be decorated, you need to take a
|
|
different approach.
|
|
|
|
.. _decorating-class-based-views:
|
|
|
|
Decorating the class
|
|
--------------------
|
|
|
|
To decorate every instance of a class-based view, you need to decorate
|
|
the class definition itself. To do this you apply the decorator to the
|
|
:meth:`~django.views.generic.base.View.dispatch` method of the class.
|
|
|
|
A method on a class isn't quite the same as a standalone function, so
|
|
you can't just apply a function decorator to the method -- you need to
|
|
transform it into a method decorator first. The ``method_decorator``
|
|
decorator transforms a function decorator into a method decorator so
|
|
that it can be used on an instance method. For example::
|
|
|
|
from django.contrib.auth.decorators import login_required
|
|
from django.utils.decorators import method_decorator
|
|
from django.views.generic import TemplateView
|
|
|
|
class ProtectedView(TemplateView):
|
|
template_name = 'secret.html'
|
|
|
|
@method_decorator(login_required)
|
|
def dispatch(self, *args, **kwargs):
|
|
return super(ProtectedView, self).dispatch(*args, **kwargs)
|
|
|
|
In this example, every instance of ``ProtectedView`` will have
|
|
login protection.
|
|
|
|
.. note::
|
|
|
|
``method_decorator`` passes ``*args`` and ``**kwargs``
|
|
as parameters to the decorated method on the class. If your method
|
|
does not accept a compatible set of parameters it will raise a
|
|
``TypeError`` exception.
|