django/docs/tutorial04.txt

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

By Adrian Holovaty <holovaty@gmail.com>

This tutorial begins where `Tutorial 3`_ 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 from the last tutorial, so that the
template contains an HTML ``<form>`` element::

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

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

    <form action="/polls/{{ poll.id }}/vote/" method="post">
    {% for choice in poll.get_choice_list %}
        <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.

Now, let's create a Django view that handles the submitted data and does
something with it. Remember, in `Tutorial 3`_, we create a URLconf that
included this line::

    (r'^polls/(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.vote'),

So let's create a ``vote()`` function in ``myproject/apps/polls/views/polls.py``::

    from django.core import template_loader
    from django.core.extensions import DjangoContext as Context
    from django.models.polls import choices, polls
    from django.utils.httpwrappers import HttpResponse, HttpResponseRedirect
    from django.core.exceptions import Http404

    def vote(request, poll_id):
        try:
            p = polls.get_object(pk=poll_id)
        except polls.PollDoesNotExist:
            raise Http404
        try:
            selected_choice = p.get_choice(pk=request.POST['choice'])
        except (KeyError, choices.ChoiceDoesNotExist):
            # Redisplay the poll voting form.
            t = template_loader.get_template('polls/detail')
            c = Context(request, {
                'poll': p,
                'error_message': "You didn't select a choice.",
            })
            return HttpResponse(t.render(c))
        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('/polls/%s/results/' % p.id)

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

    * ``request.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. ``request.POST``
      values are always strings.

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

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

    * After incrementing the choice count, the code returns an
      ``HttpResponseRedirect`` rather than a normal ``HttpResponse``.
      ``HttpResponseRedirect`` takes a single argument: the URL to which the
      user will be redirected. You should leave off the "http://" and domain
      name if you can. That helps your app become portable across domains.

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

As mentioned in Tutorial 3, ``request`` is a ``HTTPRequest`` object. For more
on ``HTTPRequest`` objects, see the `request and response documentation`_.

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):
        try:
            p = polls.get_object(pk=poll_id)
        except polls.PollDoesNotExist:
            raise Http404
        t = template_loader.get_template('polls/results')
        c = Context(request, {
            'poll': p,
        })
        return HttpResponse(t.render(c))

This is almost exactly the same as the ``detail()`` view from `Tutorial 3`_.
The only difference is the template name. We'll fix this redundancy later.

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

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

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

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.

.. _request and response documentation: http://www.djangoproject.com/documentation/request_response/

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

The ``detail()`` (from `Tutorial 3`_) 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.

.. 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.py URLconf. It looks like this, according to the tutorial
so far::

    from django.conf.urls.defaults import *

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

Change it like so::

    from django.conf.urls.defaults import *

    info_dict = {
        'app_label': 'polls',
        'module_name': 'polls',
    }

    urlpatterns = patterns('',
        (r'^$', 'django.views.generic.list_detail.object_list', info_dict),
        (r'^(?P<object_id>\d+)/$', 'django.views.generic.list_detail.object_detail', info_dict),
        (r'^(?P<object_id>\d+)/results/$', 'django.views.generic.list_detail.object_detail', dict(info_dict, template_name='polls/results')),
        (r'^(?P<poll_id>\d+)/vote/$', 'myproject.apps.polls.views.polls.vote'),
    )

We're using two generic views here: ``object_list`` and ``object_detail``.
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 which ``app_label`` and ``module_name``
      it's acting on. Thus, we've defined ``info_dict``, a dictionary that's
      passed to each of the generic views via the third parameter to the URL
      tuples.

    * The ``object_detail`` generic view expects that the ID value captured
      from the URL is called ``"object_id"``, so we've changed ``poll_id`` to
      ``object_id`` for the generic views.

By default, the ``object_detail`` generic view uses a template called
``<app_label>/<module_name>_detail``. In our case, it'll use the template
``"polls/polls_detail"``. Thus, rename your ``polls/detail.html`` template to
``polls/polls_detail.html``, and change the ``template_loader.get_template()``
line in ``vote()``.

Similarly, the ``object_list`` generic view uses a template called
``<app_label>/<module_name>_list``. Thus, rename ``polls/index.html`` to
``polls/polls_list.html``.

Because we have more than one entry in the URLconf that uses ``object_detail``
for the polls app, we manually specify a template name for the results view:
``template_name='polls/results'``. Otherwise, both views would use the same
template. Note that we use ``dict()`` to return an altered dictionary in place.

The generic views pass ``object`` and ``object_list`` to their templates, so
change your templates so that ``latest_poll_list`` becomes ``object_list`` and
``poll`` becomes ``object``.

Finally, you can delete the ``index()``, ``detail()`` and ``results()`` views
from ``polls/views/polls.py``. We don't need them anymore.

For full details on generic views, see the `generic views documentation`_.

.. _generic views documentation: http://www.djangoproject.com/documentation/generic_views/

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

The tutorial ends here for the time being. But check back soon for the next
installments:

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

.. _Tutorial 3: http://www.djangoproject.com/documentation/tutorial3/