=====================================
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:

.. snippet:: html+django
    :filename: polls/templates/polls/detail.html

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

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

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

A quick rundown:

* The above template displays a radio button for each question choice. The
  ``value`` of each radio button is the associated question 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=#`` where # is the ID of the selected choice. This is the
  basic concept of HTML forms.

* We set the form's ``action`` to ``{% url 'polls:vote' question.id %}``, 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
  :ttag:`{% csrf_token %}<csrf_token>` template tag.

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:

.. snippet::
    :filename: polls/urls.py

    url(r'^(?P<question_id>[0-9]+)/vote/$', views.vote, name='vote'),

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

.. snippet::
    :filename: polls/views.py

    from django.shortcuts import get_object_or_404, render
    from django.http import HttpResponseRedirect, HttpResponse
    from django.urls import reverse

    from .models import Choice, Question
    # ...
    def vote(request, question_id):
        question = get_object_or_404(Question, pk=question_id)
        try:
            selected_choice = question.choice_set.get(pk=request.POST['choice'])
        except (KeyError, Choice.DoesNotExist):
            # Redisplay the question voting form.
            return render(request, 'polls/detail.html', {
                'question': question,
                '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('polls:results', args=(question.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 question 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.urls.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 :doc:`Tutorial 3 </intro/tutorial03>`,
  this :func:`~django.urls.reverse` call will return a string like
  ::

    '/polls/3/results/'

  where the ``3`` is the value of ``question.id``. This redirected URL will
  then call the ``'results'`` view to display the final page.

As mentioned in :doc:`Tutorial 3 </intro/tutorial03>`, ``request`` is an
: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 question, the ``vote()`` view redirects to the results
page for the question. Let's write that view:

.. snippet::
    :filename: polls/views.py

    from django.shortcuts import get_object_or_404, render


    def results(request, question_id):
        question = get_object_or_404(Question, pk=question_id)
        return render(request, 'polls/results.html', {'question': question})

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 ``polls/results.html`` template:

.. snippet:: html+django
    :filename: polls/templates/polls/results.html

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

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

    <a href="{% url 'polls:detail' question.id %}">Vote again?</a>

Now, go to ``/polls/1/`` in your browser and vote in the question. 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.

.. note::
    The code for our ``vote()`` view does have a small problem. It first gets
    the ``selected_choice`` object from the database, then computes the new
    value of ``votes``, and then saves it back to the database. If two users of
    your website try to vote at *exactly the same time*, this might go wrong:
    The same value, let's say 42, will be retrieved for ``votes``. Then, for
    both users the new value of 43 is computed and saved, but 44 would be the
    expected value.

    This is called a *race condition*. If you are interested, you can read
    :ref:`avoiding-race-conditions-using-f` to learn how you can solve this
    issue.

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

The ``detail()`` (from :doc:`Tutorial 3 </intro/tutorial03>`) and ``results()``
views are very simple -- and, as mentioned above, redundant. The ``index()``
view, 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. Introduce new views based on Django's generic 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.

Amend URLconf
-------------

First, open the ``polls/urls.py`` URLconf and change it like so:

.. snippet::
    :filename: polls/urls.py

    from django.conf.urls import url

    from . import views

    app_name = 'polls'
    urlpatterns = [
        url(r'^$', views.IndexView.as_view(), name='index'),
        url(r'^(?P<pk>[0-9]+)/$', views.DetailView.as_view(), name='detail'),
        url(r'^(?P<pk>[0-9]+)/results/$', views.ResultsView.as_view(), name='results'),
        url(r'^(?P<question_id>[0-9]+)/vote/$', views.vote, name='vote'),
    ]

Note that the name of the matched pattern in the regexes of the second and third
patterns has changed from ``<question_id>`` to ``<pk>``.

Amend views
-----------

Next, we're going to remove our old ``index``, ``detail``, and ``results``
views and use Django's generic views instead. To do so, open the
``polls/views.py`` file and change it like so:

.. snippet::
    :filename: polls/views.py

    from django.shortcuts import get_object_or_404, render
    from django.http import HttpResponseRedirect
    from django.urls import reverse
    from django.views import generic

    from .models import Choice, Question


    class IndexView(generic.ListView):
        template_name = 'polls/index.html'
        context_object_name = 'latest_question_list'

        def get_queryset(self):
            """Return the last five published questions."""
            return Question.objects.order_by('-pub_date')[:5]


    class DetailView(generic.DetailView):
        model = Question
        template_name = 'polls/detail.html'


    class ResultsView(generic.DetailView):
        model = Question
        template_name = 'polls/results.html'


    def vote(request, question_id):
        ... # same as above, no changes needed.

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`` attribute.

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

By default, the :class:`~django.views.generic.detail.DetailView` generic
view uses a template called ``<app name>/<model name>_detail.html``.
In our case, it would use the template ``"polls/question_detail.html"``. The
``template_name`` attribute 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.detail.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 ``question`` and ``latest_question_list``
context variables. For ``DetailView`` the ``question`` variable is provided
automatically -- since we're using a Django model (``Question``), Django
is able to determine an appropriate name for the context variable.
However, for ListView, the automatically generated context variable is
``question_list``. To override this we provide the ``context_object_name``
attribute, specifying that we want to use ``latest_question_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.

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/class-based-views/index>`.

When you're comfortable with forms and generic views, read :doc:`part 5 of this
tutorial</intro/tutorial05>` to learn about testing our polls app.
