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Code Issues 32 Releases Wiki Activity

Added documentation about different results according to order of annotate() and filter() in a query.

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SirAbhi13 2022-11-08 23:45:45 +05:30
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docs/topics/db/aggregation.txt
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@ -425,6 +425,27 @@ publisher's that have at least one book with a rating exceeding 3.0. The second
query asks for the average of a publisher's book's ratings for only those query asks for the average of a publisher's book's ratings for only those
ratings exceeding 3.0. ratings exceeding 3.0.
Using multivalued filtering against a previously annotated aggregation might
still yield duplicate results.
Given:
* Publisher A has two books with each having 100 pages.
Here's an example demonstrating this behavior::
>>> a = Publisher.objects.annotate(total_pages(Sum("books__pages"))
>>> a.total_pages
200
>>> b = Publisher.objects.annotate(Sum("books__pages")).filter(books__in=Books.objects.all())
>>> b.total_pages
400
This is behavior is caused because ``filter()`` calls won't reuse multivalued
JOINS generated by previous ``annotate()`` or ``filter`` calls. To prevent issues,
you should filter after annotations (not the other way around) or use
``Aggregate.filter``.
It's difficult to intuit how the ORM will translate complex querysets into SQL It's difficult to intuit how the ORM will translate complex querysets into SQL
queries so when in doubt, inspect the SQL with ``str(queryset.query)`` and queries so when in doubt, inspect the SQL with ``str(queryset.query)`` and
write plenty of tests. write plenty of tests.