#25173 closed Bug (duplicate)
multi-table inheritance: accessing child object after select_related('child') causes extra query
Reported by: | Sebastian Illing | Owned by: | nobody |
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Component: | Database layer (models, ORM) | Version: | 1.8 |
Severity: | Normal | Keywords: | ORM, select_related, model inheritance |
Cc: | V.P. | Triage Stage: | Accepted |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description (last modified by )
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
Attachments (1)
Change History (8)
comment:1 by , 9 years ago
comment:2 by , 9 years ago
obj = A.objects.select_related('b', 'field_fk).first()
- yes there is only one query
obj = obj.b print (obj.a_ptr.field_fk)
- this also results in one query.
But unfortunately I can't do that in my actual use case.
comment:3 by , 9 years ago
Triage Stage: | Unreviewed → Accepted |
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As Shai said, it's probably worth looking into fixing this.
Attached is a reproduction testcase for this issue.
comment:4 by , 9 years ago
Description: | modified (diff) |
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Resolution: | → duplicate |
Status: | new → closed |
Summary: | Model inheritance and select_related → multi-table inheritance: accessing child object after select_related('child') causes extra query |
Looks like a duplicate of #16043.
comment:5 by , 12 months ago
Cc: | added |
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Resolution: | duplicate |
Status: | closed → new |
follow-up: 7 comment:6 by , 12 months ago
Resolution: | → duplicate |
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Status: | new → closed |
Please provide a reproducible scenario before reopening a ticket. Tests provided by Baptist pass on the current main
branch (and 3.2).
comment:7 by , 12 months ago
Mariusz Felisiak, sorry, it looks like I had this behavior because of another issue in code. Now I cannot reproduce it, everything works fine.
Could you please verify the following:
but I'd agree that, even if it works, that's a workaround for something that's worth solving.