#26211: prefetch_related with Prefetch with queryset with explicit Ordering
Ignored
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Reporter: cancan101 | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 1.9
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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Given these models:
{{{
class Parent(models.Model):
pass
class Child(models.Model):
saved_dt = models.DateTimeField(auto_now_add=True)
parent = models.ForeignKey(Parent)
}}}
I am executing this query (for each `Parent` I want to get the `child_set`
in sorted order):
{{{
[m.child_set.order_by('saved_dt') for m in
Parent.objects.prefetch_related(Prefetch('child_set',
Child.objects.order_by('saved_dt'))).all()]
}}}
I would expect that to be two SQL queries, but in reality I see `N+2`
where `N` is the number of `Child`s:
{{{
SELECT "prefetch_parent"."id" FROM "prefetch_parent"; args=()
SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
"prefetch_child"."parent_id" FROM "prefetch_child" WHERE
"prefetch_child"."parent_id" IN (1, 2) ORDER BY
"prefetch_child"."saved_dt" ASC; args=(1, 2)
SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
"prefetch_child"."parent_id" FROM "prefetch_child" WHERE
"prefetch_child"."parent_id" = 1 ORDER BY "prefetch_child"."saved_dt" ASC
LIMIT 21; args=(1,)
SELECT "prefetch_child"."id", "prefetch_child"."saved_dt",
"prefetch_child"."parent_id" FROM "prefetch_child" WHERE
"prefetch_child"."parent_id" = 2 ORDER BY "prefetch_child"."saved_dt" ASC
LIMIT 21; args=(2,)
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/26211>
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