mirrors/django
1
0
Fork 0
mirror of https://github.com/django/django.git synced 2025-11-01 20:31:40 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 4

Fixed #2705: added a select_for_update() clause to querysets.

Browse source 
A number of people worked on this patch over the years -- Hawkeye, Colin Grady,
KBS, sakyamuni, anih, jdemoor, and Issak Kelly. Thanks to them all, and
apologies if I missed anyone.

Special thanks to Dan Fairs for picking it up again at the end and seeing this
through to commit.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16058 bcc190cf-cafb-0310-a4f2-bffc1f526a37
This commit is contained in:
Jacob Kaplan-Moss 2011-04-20 20:42:07 +00:00
parent 99c1794427
commit 8f0f73c7b8
14 changed files with 375 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

40
docs/ref/models/querysets.txt
View file

@ -966,6 +966,46 @@ For example::
# queries the database with the 'backup' alias
>>> Entry.objects.using('backup')
select_for_update
~~~~~~~~~~~~~~~~~
.. method:: select_for_update(nowait=False)
.. versionadded:: 1.4
Returns a queryset that will lock rows until the end of the transaction,
generating a ``SELECT ... FOR UPDATE`` SQL statement on supported databases.
For example::
entries = Entry.objects.select_for_update().filter(author=request.user)
All matched entries will be locked until the end of the transaction block,
meaning that other transactions will be prevented from changing or acquiring
locks on them.
Usually, if another transaction has already acquired a lock on one of the
selected rows, the query will block until the lock is released. If this is
not the behaviour you want, call ``select_for_update(nowait=True)``. This will
make the call non-blocking. If a conflicting lock is already acquired by
another transaction, ``django.db.utils.DatabaseError`` will be raised when
the queryset is evaluated.
Note that using ``select_for_update`` will cause the current transaction to be
set dirty, if under transaction management. This is to ensure that Django issues
a ``COMMIT`` or ``ROLLBACK``, releasing any locks held by the ``SELECT FOR
UPDATE``.
Currently, the ``postgresql_psycopg2``, ``oracle``, and ``mysql``
database backends support ``select_for_update()``. However, MySQL has no
support for the ``nowait`` argument.
Passing ``nowait=True`` to ``select_for_update`` using database backends that
do not support ``nowait``, such as MySQL, will cause a ``DatabaseError`` to be
raised. This is in order to prevent code unexpectedly blocking.
Using ``select_for_update`` on backends which do not support
``SELECT ... FOR UPDATE`` (such as SQLite) will have no effect.
Methods that do not return QuerySets
------------------------------------