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[2.1.x] Clarified when QuerySet.select_for_update() locks. 

Backport of d093e01ec0 from master.
This commit is contained in:
ovalseven8 2018-09-22 15:02:20 +02:00 committed by Tim Graham
parent 173e242d71
commit 9e5f4bcedb
1 changed files with 10 additions and 4 deletions
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14
docs/ref/models/querysets.txt
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@ -1664,11 +1664,17 @@ generating a ``SELECT ... FOR UPDATE`` SQL statement on supported databases.
For example::
entries = Entry.objects.select_for_update().filter(author=request.user)
from django.db import transaction
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.
entries = Entry.objects.select_for_update().filter(author=request.user)
with transaction.atomic():
for entry in entries:
...
When the queryset is evaluated (``for entry in entries`` in this case), 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