mirror of https://github.com/django/django.git
Removed distracting note from tutorial 4.
The note on a possible race condition is inappropriate in this tutorial setting. To quote Diátaxis: > Your job is to guide the learner to a successful conclusion. There > may be many interesting diversions along the way … - ignore them. Co-Authored-By: Ryan Hiebert <ryan@ryanhiebert.com>
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@ -74,6 +74,7 @@ create a real version. Add the following to ``polls/views.py``:
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.. code-block:: python
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:caption: ``polls/views.py``
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from django.db.models import F
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from django.http import HttpResponse, HttpResponseRedirect
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from django.shortcuts import get_object_or_404, render
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from django.urls import reverse
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@ -97,7 +98,7 @@ create a real version. Add the following to ``polls/views.py``:
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},
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)
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else:
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selected_choice.votes += 1
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selected_choice.votes = F("votes") + 1
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selected_choice.save()
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# Always return an HttpResponseRedirect after successfully dealing
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# with POST data. This prevents data from being posted twice if a
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@ -123,6 +124,9 @@ This code includes a few things we haven't covered yet in this tutorial:
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:exc:`KeyError` and redisplays the question form with an error
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message if ``choice`` isn't given.
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* ``F("votes") + 1`` :ref:`instructs the database
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<avoiding-race-conditions-using-f>` to increase the vote count by 1.
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* After incrementing the choice count, the code returns an
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:class:`~django.http.HttpResponseRedirect` rather than a normal
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:class:`~django.http.HttpResponse`.
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@ -190,19 +194,6 @@ Now, go to ``/polls/1/`` in your browser and vote in the question. You should se
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results page that gets updated each time you vote. If you submit the form
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without having chosen a choice, you should see the error message.
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.. note::
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The code for our ``vote()`` view does have a small problem. It first gets
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the ``selected_choice`` object from the database, then computes the new
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value of ``votes``, and then saves it back to the database. If two users of
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your website try to vote at *exactly the same time*, this might go wrong:
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The same value, let's say 42, will be retrieved for ``votes``. Then, for
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both users the new value of 43 is computed and saved, but 44 would be the
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expected value.
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This is called a *race condition*. If you are interested, you can read
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:ref:`avoiding-race-conditions-using-f` to learn how you can solve this
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issue.
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Use generic views: Less code is better
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======================================
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