diff -r 261778de26ff -r 620f9b141567 thirdparty/google_appengine/lib/django/docs/tutorial04.txt --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/thirdparty/google_appengine/lib/django/docs/tutorial04.txt Tue Aug 26 21:49:54 2008 +0000 @@ -0,0 +1,259 @@ +===================================== +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 ``
`` element:: + +

{{ poll.question }}

+ + {% if error_message %}

{{ error_message }}

{% endif %} + + + {% for choice in poll.choice_set.all %} + +
+ {% endfor %} + +
+ +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\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:: + +

{{ poll.question }}

+ + + +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'^$', 'index'), + (r'^(?P\d+)/$', 'detail'), + (r'^(?P\d+)/results/$', 'results'), + (r'^(?P\d+)/vote/$', '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'^$', 'django.views.generic.list_detail.object_list', info_dict), + (r'^(?P\d+)/$', 'django.views.generic.list_detail.object_detail', info_dict), + (r'^(?P\d+)/results/$', 'django.views.generic.list_detail.object_detail', dict(info_dict, template_name='polls/results.html')), + (r'^(?P\d+)/vote/$', '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 +``/_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 +``/_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/