| ===================================== |
| 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/ |