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Fixed #7060 -- Added a note about race conditions to the tutorial

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Raphael Michel 2015-06-04 15:09:28 +02:00 committed by Tim Graham
parent 19e67c6cd1
commit 3e9b5bfd9c
1 changed files with 13 additions and 0 deletions
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docs/intro/tutorial04.txt
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@ -179,6 +179,19 @@ Now, go to ``/polls/1/`` in your browser and vote in the question. You should se
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 Web site 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
======================================