#30493: GenericRelation and prefetch_related: wrong caching with cyclic
prefetching
-------------------------------------+-------------------------------------
Reporter: Finn | Owner: nobody
Stutzenstein |
Type: Bug | Status: new
Component: | Version: 2.2
contrib.contenttypes | Keywords: GenericRelation
Severity: Normal | prefetch_related
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Hello @all!
I encountered an issue with GenericRelations. Here is an example to
reproduce this issue:
https://github.com/FinnStutzenstein/GenericRelatedPrefetch
Just do a migrate and runserver. The code showing the error is started
automatically in main/apps.py. Please start with --noreload not to have
the output twice.
Whats the problem?
I have a generic model (Tag) that have a {{{content_object}}}. Then there
are multiple (in the example 2) models, Book and CD, that have exactly one
tag assigned. In the real application (OpenSlides) this invariant is
ensured in other places; in the example the objects are created in a way,
that this invariant holds.
All these content objects have a property {{{tag}}}, that should return
the one assigned tag. This is done by adding a helper field
{{{tags=GenericRelation(Tag)}}} and the property accesses
{{{self.tags.all()[0]}}}. The {{{.all()[0]}}} instead of a simple
{{{.get()}}} is required for the prefetching to work. See main/models.py
in the example.
Now all tags should be loaded because in OpenSlides they would be
serialized. For each tag the {{{content_object}}} is accessed as well as
{{{content_object.tag}}}. Because this would result in many DB queries (in
the real application about 10000 Tags, and in sum 10000 content objects)
the models are prefetched with:
{{{Tag.objects.prefetch_related("content_object",
"content_object__tag")}}} (Note: The executed code is in main/apps.py).
This results in a constant amount of queries (4 in this case) instead of
something proportional to the amount of objects. In the example you can
set {{{N}}}, the amount of objects created, to a higher amount to verify,
that the amount of queries stays constant.
What is expected: If I have a tag {{{tag}}}, {{{tag.content_object.tag}}}
should be equal to {{{tag}}}.
Output from the example (with N=1):
{{{
Got 'Tag to book0':
-the content object: Book0
-the content objects tag (should be the same as 'Tag to book0'!):Tag
to book0
Got 'Tag to cd0':
-the content object: CD0
-the content objects tag (should be the same as 'Tag to cd0'!):Tag to
book0
}}}
This is not the case: 'Tag to cd1' -> 'cd1' -> 'Tag to book1'.
I tracked this a bit showing, that {{{_prefetched_objects_cache}}} holds
the wrong value, which is accessed through {{{.all()}}} ->
{{{.get_queryset()}}} where the cached/prefetched result is taken.
Thanks!
--
Ticket URL: <https://code.djangoproject.com/ticket/30493>
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