#25173: multi-table inheritance: accessing child object after
select_related('child') causes extra query
-------------------------------------+-------------------------------------
Reporter: illing2005 | Owner: nobody
Type: Bug | Status: closed
Component: Database layer | Version: 1.8
(models, ORM) |
Severity: Normal | Resolution: duplicate
Keywords: ORM, | Triage Stage: Accepted
select_related, model inheritance |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by timgraham):
* status: new => closed
* resolution: => duplicate
Old description:
> I have the following in models and run into strange behavior when i use
> select_related and model inheritance:
>
> Models:
> {{{
> class A(models.Model):
> field_fk = models.ForeignKey('C')
>
> class B(A):
> fields_b = models.CharField(max_length=255)
>
> class C(models.Model):
> field_c = models.CharField(max_length=255)
> }}}
>
> So A has a foreign key to C and B inherits from A. Now I want to query A
> downcast it to B and read the relationship to C. To minimize sql queries
> I use select_related:
>
> {{{
> obj = A.objects.select_related('b', 'field_fk).first()
> obj = obj.b
> print(obj.field_fk) # this prints "C object"
> }}}
>
> Because I use select_related this should result in just one query. But
> somehow the information is lost during downcasting and I get to sql
> queries:
>
> {{{
> SELECT ••• FROM "base_a" INNER JOIN "base_c" ON
> ( "base_a"."field_fk_id" = "base_c"."id" ) LEFT OUTER JOIN "base_b"
> ON
> ( "base_a"."id" = "base_b"."a_ptr_id" ) ORDER BY "base_a"."id" ASC
> LIMIT 1
>
> SELECT ••• FROM "base_c" WHERE "base_c"."id" = 1
> }}}
>
> So in the first query looks fine. But I am surprised that I get a second
> query. Is this a bug in django's ORM or am I doing something wrong?
>
> I also posted this at stackoverflow, but I am quite sure that this is a
> bug.
> http://stackoverflow.com/questions/31628588/django-model-inheritance-and-
> select-related
New description:
I have the following in models and run into strange behavior when i use
select_related and model inheritance:
Models:
{{{
class A(models.Model):
field_fk = models.ForeignKey('C')
class B(A):
fields_b = models.CharField(max_length=255)
class C(models.Model):
field_c = models.CharField(max_length=255)
}}}
So A has a foreign key to C and B inherits from A. Now I want to query A
downcast it to B and read the relationship to C. To minimize sql queries I
use select_related:
{{{
obj = A.objects.select_related('b', 'field_fk).first()
obj = obj.b
print(obj.field_fk) # this prints "C object"
}}}
Because I use select_related this should result in just one query. But
somehow the information is lost during downcasting and I get two sql
queries:
{{{
SELECT ••• FROM "base_a" INNER JOIN "base_c" ON
( "base_a"."field_fk_id" = "base_c"."id" ) LEFT OUTER JOIN "base_b"
ON
( "base_a"."id" = "base_b"."a_ptr_id" ) ORDER BY "base_a"."id" ASC
LIMIT 1
SELECT ••• FROM "base_c" WHERE "base_c"."id" = 1
}}}
So in the first query looks fine. But I am surprised that I get a second
query. Is this a bug in django's ORM or am I doing something wrong?
I also posted this at stackoverflow, but I am quite sure that this is a
bug.
http://stackoverflow.com/questions/31628588/django-model-inheritance-and-
select-related
--
Comment:
Looks like a duplicate of #16043.
--
Ticket URL: <https://code.djangoproject.com/ticket/25173#comment:4>
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