=====================================

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 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 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('/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):

        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/

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),

        (r'^(?P<object_id>\d+)/results/$', 'django.views.generic.list_detail.object_detail', dict(info_dict, template_name='polls/results.html')),

        (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.

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``.

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

.. _Tutorial 3: ../tutorial3/