#22757: prefetch_related isn't as effecient as it could be with
GenericForeignKey
and proxy models
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Reporter: gwahl@… | Owner: nobody
Type: Uncategorized | Status: new
Component: contrib.contenttypes | Version: 1.6
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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`prefetch_related` is able to prefetch across a GenericForiegnKey.
However, it does more queries than necessary when using proxy models.
Our models:
{{{#!python
from django.db import models
from django.contrib.contenttypes.generic import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
class Parent(models.Model):
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
child = GenericForeignKey('content_type', 'object_id',
for_concrete_model=False)
class Child(models.Model):
pass
class ProxiedChild(Child):
class Meta:
proxy = True
}}}
Now create some instances
{{{
>>> child = Child.objects.create()
>>> proxied_child = ProxiedChild.objects.create()
>>> p1 = Parent.objects.create(child=child)
>>> p2 = Parent.objects.create(child=proxied_child)
}}}
And query using prefetch_related:
{{{
>>> Parent.objects.all().prefetch_related('child')
SELECT "foo_parent"."id", "foo_parent"."content_type_id",
"foo_parent"."object_id" FROM "foo_parent" LIMIT 21
SELECT "foo_child"."id" FROM "foo_child" WHERE "foo_child"."id" IN (3)
SELECT "foo_child"."id" FROM "foo_child" WHERE "foo_child"."id" IN (2)
}}}
This is doing 3 queries instead of the expected 2. The two queries for the
`foo_child` table should be combined to be `"foo_child"."id" IN (2, 3)`
--
Ticket URL: <https://code.djangoproject.com/ticket/22757>
Django <https://code.djangoproject.com/>
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