Filtering by related objects causes unnecessary extra db hits when using model inheritance

[Johannes Dollinger]

The failing test below illustrates the issue.

from django.test import TestCase
from django.db import models

class A(models.Model): pass
class B(A): pass

class R(models.Model):
     b = models.ForeignKey(B, related_name="r")


class T(TestCase):
     def test(self):
        b = B.objects.create()
        r = R.objects.create(b=b)
        # Fails: produces 2 queries
        self.assertNumQueries(1, lambda: list(R.objects.filter(b=b)))
        # Fails: produces 2 queries
        self.assertNumQueries(1, lambda: list(b.r.all())

The extra queries seem to be generated by RelatedField._pk_trace().

[Preston Holmes]

Why were code comments deleted in patch? They still seem valid, otherwise they should be updated not deleted.

[Tim Graham]

Looks like this was fixed in Django 1.6 with 97774429aeb54df4c09895c07cd1b09e70201f7d.

​PR to add a regression test.

[Tim Graham <timograham@…>]

In fa2e1e63:

Refs #15844 -- Added tests for multi-table inheritance related object filtering efficiency.

Fixed in 97774429aeb54df4c09895c07cd1b09e70201f7d.