Fixed #5550: Documented the context used by the default view for 404 and 500 errors.

[django.git] / docs / tutorial04.txt

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

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 ("polls/detail.html") 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.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.

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

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

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

    from django.shortcuts import get_object_or_404, render_to_response
    from django.http import HttpResponseRedirect
    from django.core.urlresolvers import reverse
    from mysite.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.",
            })
        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('mysite.polls.views.results', args=(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 (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
      ``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 ``reverse()`` function in the ``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 ``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).

      For more information about ``reverse()``, see the `URL dispatcher`_
      documentation.

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):
        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 `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.choice_set.all %}
        <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: ../request_response/
.. _URL dispatcher: ../url_dispatch#reverse

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

    from django.conf.urls.defaults import *

    urlpatterns = patterns('mysite.polls.views',
        (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 *
    from mysite.polls.models import Poll

    info_dict = {
        'queryset': Poll.objects.all(),
    }

    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),
        url(r'^(?P<object_id>\d+)/results/$', 'django.views.generic.list_detail.object_detail', dict(info_dict, template_name='polls/results.html'), 'poll_results'),
        (r'^(?P<poll_id>\d+)/vote/$', 'mysite.polls.views.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 what data it will be acting upon. This
      data is provided in a dictionary. The ``queryset`` key in this dictionary
      points to the list of objects to be manipulated by the generic view.

    * The ``object_detail`` generic view expects the ID value captured
      from the URL to be called ``"object_id"``, so we've changed ``poll_id`` to
      ``object_id`` 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
      `naming URL patterns`_ for information). We're also using the `url()`_
      function from ``django.conf.urls.defaults`` here. It's a good habit to
      use ``url()`` when you are providing a pattern name like this.

.. _naming URL patterns: ../url_dispatch/#naming-url-patterns
.. _url(): ../url_dispatch/#url

By default, the ``object_detail`` generic view uses a template called
``<app name>/<model name>_detail.html``. In our case, it'll use the template
``"polls/poll_detail.html"``. Thus, rename your ``polls/detail.html`` template to
``polls/poll_detail.html``, and change the ``render_to_response()`` line in
``vote()``.

Similarly, the ``object_list`` generic view uses a template called
``<app name>/<model name>_list.html``. Thus, rename ``polls/index.html`` to
``polls/poll_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.html'``. Otherwise, both views would use the same
template. Note that we use ``dict()`` to return an altered dictionary in place.

.. note:: ``all()`` is lazy

    It might look a little frightening to see ``Poll.objects.all()`` being used
    in a detail view which only needs one ``Poll`` object, but don't worry;
    ``Poll.objects.all()`` is actually a special object called a ``QuerySet``,
    which is "lazy" and doesn't hit your database until it absolutely has to. By
    the time the database query happens, the ``object_detail`` generic view will
    have narrowed its scope down to a single object, so the eventual query will
    only select one row from the database. 
    
    If you'd like to know more about how that works, The Django database API
    documentation `explains the lazy nature of QuerySet objects`_.

.. _explains the lazy nature of QuerySet objects: ../db-api/#querysets-are-lazy

In previous parts of the tutorial, the templates have been provided with a context
that contains the ``poll`` and ``latest_poll_list`` context variables. However,
the generic views provide the variables ``object`` and ``object_list`` as context.
Therefore, you need to change your templates to match the new context variables.
Go through your templates, and modify any reference to ``latest_poll_list`` to
``object_list``, and change any reference to ``poll`` to ``object``.

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 ``vote()`` view is still required. However, it must be modified to match
the new templates and context variables. Change the template call from
``polls/detail.html`` to ``polls/poll_detail.html``, and pass ``object`` in the
context instead of ``poll``.

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 ``reverse()`` function to avoid
hard-coding our URLs. Now that we've switched to a generic view, we'll need to
change the ``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 `generic views documentation`_.

.. _generic views 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

In the meantime, you can read through the rest of the `Django documentation`_
and start writing your own applications.

.. _Tutorial 3: ../tutorial03/
.. _Django documentation: http://www.djangoproject.com/documentation/