select_related with nested fields affects query result

[kriomant@…]

I belive select_related should never affect query result, but only optimize referenced objects loading. However, I've found a case where query result is changed.

models.py

from django.db import models

class Thing(models.Model):
        name = models.CharField(max_length=255)

        def __unicode__(self):
                return self.name

class User(models.Model):
        login = models.CharField(max_length=255)

class Lock(models.Model):
        description = models.CharField(max_length=255)

        # Only one lock per thing is allowed.
        thing = models.OneToOneField(Thing)

        user = models.ForeignKey(User)

tests.py

from django.test import TestCase

from reproduce.models import *

class SimpleTest(TestCase):
        def setUp(self):
                user = User.objects.create(login='james')

                # Unlocked thing.
                self.unlocked_thing = Thing.objects.create(name='unlocked')

                # Locked thing.
                self.locked_thing = Thing.objects.create(name='locked')
                Lock.objects.create(thing=self.locked_thing, description='lock', user=user)

        def test_simple_query(self):
                things = Thing.objects.all()
                self.assertEqual(set(things), set([self.unlocked_thing, self.locked_thing]))

        def test_select_related(self):
                things = Thing.objects.select_related('lock')
                self.assertEqual(set(things), set([self.locked_thing, self.unlocked_thing]))

        def test_deep_select_related(self):
                things = Thing.objects.select_related('lock', 'lock__user')
                # This fails.
                self.assertEqual(set(things), set([self.locked_thing, self.unlocked_thing]))

./manage.py test reproduce produces:

Creating test database for alias 'default'...
F..
======================================================================
FAIL: test_deep_select_related (reproduce.tests.SimpleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/.../select_related_bug/reproduce/tests.py", line 27, in test_deep_select_related
    self.assertEqual(set(things), set([self.locked_thing, self.unlocked_thing]))
AssertionError: Items in the second set but not the first:
<Thing: unlocked>

----------------------------------------------------------------------
Ran 3 tests in 0.006s

FAILED (failures=1)
Destroying test database for alias 'default'...

[Malcolm Tredinnick]

The initial observation is correct: select_related() is only an optimisation and should never change the results returned at the outermost object level (it will, of course, select extra lower-level objects).

[Tim Graham]

Fixed in a193372753ad9d1d15ad5e2d6d06bbc07ca3f433