#35697: Filtering on a ForeignKey relation with the field it relates to is not
possible
-------------------------------------+-------------------------------------
Reporter: Petter Friberg | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 5.0 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
My use case is that I have a `ForeignKey` relation with `constraint=False`
and then want to use `rel_id=models.F("rel__pk")` to ensure that the value
indeed exists in the related table.
Example:
{{{
#!python
from django.db import models
class A(models.Model):
...
class B(models.Model):
a = models.ForeignKey(A, on_delete=models.CASCADE, constraint=False)
B.objects.filter(a_id=models.F("a__pk"))
}}}
Here I would (optimally) want/expect the SQL to look like, though I would
suspect that would have to be quite special cased:
{{{
#!sql
SELECT "app_b"."id", "app_b"."a_id" FROM "app_b" INNER JOIN "app_a" ON
("app_b"."a_id" = "app_a"."id")
}}}
What I'm actually getting, I'm guessing that Django tries to be a bit too
"smart" when resolving `"a__pk"` here:
{{{
#!sql
SELECT "app_b"."id", "app_b"."a_id" FROM "app_b" WHERE "app_b"."a_id" =
("app_b"."a_id")
}}}
Though I can get my `INNER JOIN` by adding a `.select_related` call. If I
also pair that with calling e.g. `.only` I can minimise the additional
impact of the join. But I still thought it was worth filing this as I
figured it was quite unexpected that `"a__pk"` was resolved to
`"app_b"."a_id"` in this situation.
Note: I get the same SQL output when passing the primary key field
explicitly, `F("a__id")` in this case, instead of `pk`.
--
Ticket URL: <https://code.djangoproject.com/ticket/35697>
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