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Fixed #27403 -- Doc'd that QuerySet.prefetch_related() doesn't guarantee transactional consistency.

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Added a note about the potential race condition in prefetch_related()
that could produce an inconsistent result, one that does not correspond
to any point in the database history.
This commit is contained in:
Tim Bell 2023-10-20 14:37:19 -04:00 committed by Mariusz Felisiak
parent 64060d1c17
commit ee104251c4
1 changed files with 13 additions and 0 deletions
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docs/ref/models/querysets.txt
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@ -1211,6 +1211,19 @@ results; these ``QuerySets`` are then used in the ``self.toppings.all()`` calls.
The additional queries in ``prefetch_related()`` are executed after the The additional queries in ``prefetch_related()`` are executed after the
``QuerySet`` has begun to be evaluated and the primary query has been executed. ``QuerySet`` has begun to be evaluated and the primary query has been executed.
Note that there is no mechanism to prevent another database query from altering
the items in between the execution of the primary query and the additional
queries, which could produce an inconsistent result. For example, if a
``Pizza`` is deleted after the primary query has executed, its toppings will
not be returned in the additional query, and it will seem like the pizza has no
toppings:
.. code-block:: pycon
>>> Pizza.objects.prefetch_related("toppings")
# "Hawaiian" Pizza was deleted in another shell.
<QuerySet [<Pizza: Hawaiian ()>, <Pizza: Seafood (prawns, smoked salmon)>]>
If you have an iterable of model instances, you can prefetch related attributes If you have an iterable of model instances, you can prefetch related attributes
on those instances using the :func:`~django.db.models.prefetch_related_objects` on those instances using the :func:`~django.db.models.prefetch_related_objects`
function. function.