tutorial04.txt

=====================================
Writing your first Django app, part 4
=====================================

This tutorial begins where :doc:`Tutorial 3 </intro/tutorial03>` left off. We're
continuing the Web-poll application and will focus on simple form processing and
cutting down our code.

Write a simple form
===================

Let's update our poll detail template ("polls/detail.html") from the last
tutorial, so that the template contains an HTML ``<form>`` element:

.. code-block:: html+django

    <h1>{{ poll.question }}</h1>

    {% if error_message %}<p><strong>{{ error_message }}</strong></p>{% endif %}

    <form action="/polls/{{ poll.id }}/vote/" method="post">
    {% csrf_token %}
    {% for choice in poll.choice_set.all %}
        <input type="radio" name="choice" id="choice{{ forloop.counter }}" value="{{ choice.id }}" />
        <label for="choice{{ forloop.counter }}">{{ choice.choice }}</label><br />
    {% endfor %}
    <input type="submit" value="Vote" />
    </form>

A quick rundown:

    * The above template displays a radio button for each poll choice. The
      ``value`` of each radio button is the associated poll choice's ID. The
      ``name`` of each radio button is ``"choice"``. That means, when somebody
      selects one of the radio buttons and submits the form, it'll send the
      POST data ``choice=3``. This is HTML Forms 101.

    * We set the form's ``action`` to ``/polls/{{ poll.id }}/vote/``, and we
      set ``method="post"``. Using ``method="post"`` (as opposed to
      ``method="get"``) is very important, because the act of submitting this
      form will alter data server-side. Whenever you create a form that alters
      data server-side, use ``method="post"``. This tip isn't specific to
      Django; it's just good Web development practice.

    * ``forloop.counter`` indicates how many times the :ttag:`for` tag has gone
      through its loop

    * Since we're creating a POST form (which can have the effect of modifying
      data), we need to worry about Cross Site Request Forgeries.
      Thankfully, you don't have to worry too hard, because Django comes with
      a very easy-to-use system for protecting against it. In short, all POST
      forms that are targeted at internal URLs should use the ``{% csrf_token %}``
      template tag.

The ``{% csrf_token %}`` tag requires information from the request object, which
is not normally accessible from within the template context. To fix this, a
small adjustment needs to be made to the ``detail`` view, so that it looks like
the following::

    from django.template import RequestContext
    # ...
    def detail(request, poll_id):
        p = get_object_or_404(Poll, pk=poll_id)
        return render_to_response('polls/detail.html', {'poll': p},
                                   context_instance=RequestContext(request))

The details of how this works are explained in the documentation for
:ref:`RequestContext <subclassing-context-requestcontext>`.

Now, let's create a Django view that handles the submitted data and does
something with it. Remember, in :doc:`Tutorial 3 </intro/tutorial03>`, we
created a URLconf for the polls application that includes this line::

    (r'^(?P<poll_id>\d+)/vote/$', 'vote'),

We also created a dummy implementation of the ``vote()`` function. Let's
create a real version. Add the following to ``polls/views.py``::

    from django.shortcuts import get_object_or_404, render_to_response
    from django.http import HttpResponseRedirect, HttpResponse
    from django.core.urlresolvers import reverse
    from django.template import RequestContext
    from polls.models import Choice, Poll
    # ...
    def vote(request, poll_id):
        p = get_object_or_404(Poll, pk=poll_id)
        try:
            selected_choice = p.choice_set.get(pk=request.POST['choice'])
        except (KeyError, Choice.DoesNotExist):
            # Redisplay the poll voting form.
            return render_to_response('polls/detail.html', {
                'poll': p,
                'error_message': "You didn't select a choice.",
            }, context_instance=RequestContext(request))
        else:
            selected_choice.votes += 1
            selected_choice.save()
            # Always return an HttpResponseRedirect after successfully dealing
            # with POST data. This prevents data from being posted twice if a
            # user hits the Back button.
            return HttpResponseRedirect(reverse('polls.views.results', args=(p.id,)))

This code includes a few things we haven't covered yet in this tutorial:

    * :attr:`request.POST <django.http.HttpRequest.POST>` is a dictionary-like
      object that lets you access submitted data by key name. In this case,
      ``request.POST['choice']`` returns the ID of the selected choice, as a
      string. :attr:`request.POST <django.http.HttpRequest.POST>` values are
      always strings.

      Note that Django also provides :attr:`request.GET
      <django.http.HttpRequest.GET>` for accessing GET data in the same way --
      but we're explicitly using :attr:`request.POST
      <django.http.HttpRequest.POST>` in our code, to ensure that data is only
      altered via a POST call.

    * ``request.POST['choice']`` will raise :exc:`KeyError` if ``choice`` wasn't
      provided in POST data. The above code checks for :exc:`KeyError` and
      redisplays the poll form with an error message if ``choice`` isn't given.

    * After incrementing the choice count, the code returns an
      :class:`~django.http.HttpResponseRedirect` rather than a normal
      :class:`~django.http.HttpResponse`.
      :class:`~django.http.HttpResponseRedirect` takes a single argument: the
      URL to which the user will be redirected (see the following point for how
      we construct the URL in this case).

      As the Python comment above points out, you should always return an
      :class:`~django.http.HttpResponseRedirect` after successfully dealing with
      POST data. This tip isn't specific to Django; it's just good Web
      development practice.

    * We are using the :func:`~django.core.urlresolvers.reverse` function in the
      :class:`~django.http.HttpResponseRedirect` constructor in this example.
      This function helps avoid having to hardcode a URL in the view function.
      It is given the name of the view that we want to pass control to and the
      variable portion of the URL pattern that points to that view. In this
      case, using the URLconf we set up in Tutorial 3, this
      :func:`~django.core.urlresolvers.reverse` call will return a string like
      ::

        '/polls/3/results/'

      ... where the ``3`` is the value of ``p.id``. This redirected URL will
      then call the ``'results'`` view to display the final page. Note that you
      need to use the full name of the view here (including the prefix).

As mentioned in Tutorial 3, ``request`` is a :class:`~django.http.HttpRequest`
object. For more on :class:`~django.http.HttpRequest` objects, see the
:doc:`request and response documentation </ref/request-response>`.

After somebody votes in a poll, the ``vote()`` view redirects to the results
page for the poll. Let's write that view::

    def results(request, poll_id):
        p = get_object_or_404(Poll, pk=poll_id)
        return render_to_response('polls/results.html', {'poll': p})

This is almost exactly the same as the ``detail()`` view from :doc:`Tutorial 3
</intro/tutorial03>`. The only difference is the template name. We'll fix this
redundancy later.

Now, create a ``results.html`` template:

.. code-block:: html+django

    <h1>{{ poll.question }}</h1>

    <ul>
    {% for choice in poll.choice_set.all %}
        <li>{{ choice.choice }} -- {{ choice.votes }} vote{{ choice.votes|pluralize }}</li>
    {% endfor %}
    </ul>

    <a href="/polls/{{ poll.id }}/">Vote again?</a>

Now, go to ``/polls/1/`` in your browser and vote in the poll. You should see a
results page that gets updated each time you vote. If you submit the form
without having chosen a choice, you should see the error message.

Use generic views: Less code is better
======================================

The ``detail()`` (from :doc:`Tutorial 3 </intro/tutorial03>`) and ``results()``
views are stupidly simple -- and, as mentioned above, redundant. The ``index()``
view (also from Tutorial 3), which displays a list of polls, is similar.

These views represent a common case of basic Web development: getting data from
the database according to a parameter passed in the URL, loading a template and
returning the rendered template. Because this is so common, Django provides a
shortcut, called the "generic views" system.

Generic views abstract common patterns to the point where you don't even need
to write Python code to write an app.

Let's convert our poll app to use the generic views system, so we can delete a
bunch of our own code. We'll just have to take a few steps to make the
conversion. We will:

    1. Convert the URLconf.

    2. Delete some of the old, unneeded views.

    3. Fix up URL handling for the new views.

Read on for details.

.. admonition:: Why the code-shuffle?

    Generally, when writing a Django app, you'll evaluate whether generic views
    are a good fit for your problem, and you'll use them from the beginning,
    rather than refactoring your code halfway through. But this tutorial
    intentionally has focused on writing the views "the hard way" until now, to
    focus on core concepts.

    You should know basic math before you start using a calculator.

First, open the ``polls/urls.py`` URLconf. It looks like this, according to the
tutorial so far::

    from django.conf.urls.defaults import patterns, include, url

    urlpatterns = patterns('polls.views',
        url(r'^$', 'index'),
        url(r'^(?P<poll_id>\d+)/$', 'detail'),
        url(r'^(?P<poll_id>\d+)/results/$', 'results'),
        url(r'^(?P<poll_id>\d+)/vote/$', 'vote'),
    )

Change it like so::

    from django.conf.urls.defaults import *
    from django.views.generic import DetailView, ListView
    from polls.models import Poll

    urlpatterns = patterns('',
        url(r'^$',
            ListView.as_view(
                queryset=Poll.objects.order_by('-pub_date')[:5],
                context_object_name='latest_poll_list',
                template_name='polls/index.html')),
        url(r'^(?P<pk>\d+)/$',
            DetailView.as_view(
                model=Poll,
                template_name='polls/detail.html')),
        url(r'^(?P<pk>\d+)/results/$',
            DetailView.as_view(
                model=Poll,
                template_name='polls/results.html'),
            name='poll_results'),
        url(r'^(?P<poll_id>\d+)/vote/$', 'polls.views.vote'),
    )

We're using two generic views here:
:class:`~django.views.generic.list.ListView` and
:class:`~django.views.generic.detail.DetailView`. Respectively, those
two views abstract the concepts of "display a list of objects" and
"display a detail page for a particular type of object."

    * Each generic view needs to know what model it will be acting
      upon. This is provided using the ``model`` parameter.

    * The :class:`~django.views.generic.list.DetailView` generic view
      expects the primary key value captured from the URL to be called
      ``"pk"``, so we've changed ``poll_id`` to ``pk`` for the generic
      views.

    * We've added a name, ``poll_results``, to the results view so
      that we have a way to refer to its URL later on (see the
      documentation about :ref:`naming URL patterns
      <naming-url-patterns>` for information). We're also using the
      :func:`~django.conf.urls.default.url` function from
      :mod:`django.conf.urls.defaults` here. It's a good habit to use
      :func:`~django.conf.urls.defaults.url` when you are providing a
      pattern name like this.

By default, the :class:`~django.views.generic.list.DetailView` generic
view uses a template called ``<app name>/<model name>_detail.html``.
In our case, it'll use the template ``"polls/poll_detail.html"``. The
``template_name`` argument is used to tell Django to use a specific
template name instead of the autogenerated default template name. We
also specify the ``template_name`` for the ``results`` list view --
this ensures that the results view and the detail view have a
different appearance when rendered, even though they're both a
:class:`~django.views.generic.list.DetailView` behind the scenes.

Similarly, the :class:`~django.views.generic.list.ListView` generic
view uses a default template called ``<app name>/<model
name>_list.html``; we use ``template_name`` to tell
:class:`~django.views.generic.list.ListView` to use our existing
``"polls/index.html"`` template.

In previous parts of the tutorial, the templates have been provided
with a context that contains the ``poll`` and ``latest_poll_list``
context variables. For DetailView the ``poll`` variable is provided
automatically -- since we're using a Django model (``Poll``), Django
is able to determine an appropriate name for the context variable.
However, for ListView, the automatically generated context variable is
``poll_list``. To override this we provide the ``context_object_name``
option, specifying that we want to use ``latest_poll_list`` instead.
As an alternative approach, you could change your templates to match
the new default context variables -- but it's a lot easier to just
tell Django to use the variable you want.

You can now delete the ``index()``, ``detail()`` and ``results()``
views from ``polls/views.py``. We don't need them anymore -- they have
been replaced by generic views.

The last thing to do is fix the URL handling to account for the use of
generic views. In the vote view above, we used the
:func:`~django.core.urlresolvers.reverse` function to avoid
hard-coding our URLs. Now that we've switched to a generic view, we'll
need to change the :func:`~django.core.urlresolvers.reverse` call to
point back to our new generic view. We can't simply use the view
function anymore -- generic views can be (and are) used multiple times
-- but we can use the name we've given::

    return HttpResponseRedirect(reverse('poll_results', args=(p.id,)))

Run the server, and use your new polling app based on generic views.

For full details on generic views, see the :doc:`generic views documentation
</topics/http/generic-views>`.

Coming soon
===========

The tutorial ends here for the time being. Future installments of the tutorial
will cover:

    * Advanced form processing
    * Using the RSS framework
    * Using the cache framework
    * Using the comments framework
    * Advanced admin features: Permissions
    * Advanced admin features: Custom JavaScript

In the meantime, you might want to check out some pointers on :doc:`where to go
from here </intro/whatsnext>`