def test_o2o_cross_database_protection(self):
        # Create a user and profile on the default database
        alice = User.objects.db_manager('default').create_user('alice', 'alice@example.com')

        # Create a user and profile on the other database
        bob = User.objects.db_manager('other').create_user('bob', 'bob@example.com')

        # Set a one-to-one relation with an object from a different database
        alice_profile = UserProfile.objects.using('default').create(user=alice, flavor='chocolate')
        try:
            bob.userprofile = alice_profile
            self.fail("Shouldn't be able to assign across databases")
        except ValueError:
            pass

        # BUT! if you assign a FK object when the base object hasn't
        # been saved yet, you implicitly assign the database for the
        # base object.
        bob_profile = UserProfile.objects.using('other').create(user=bob, flavor='crunchy frog')

        new_bob_profile = UserProfile(flavor="spring surprise")

        charlie = User(username='charlie',email='charlie@example.com')
        charlie.set_unusable_password()

        # initially, no db assigned
        self.assertEqual(new_bob_profile._state.db, None)
        self.assertEqual(charlie._state.db, None)

        # old object comes from 'other', so the new object is set to use 'other'...
        new_bob_profile.user = bob
        charlie.userprofile = bob_profile
        self.assertEqual(new_bob_profile._state.db, 'other')
        self.assertEqual(charlie._state.db, 'other')

        # ... but it isn't saved yet
        self.assertEqual(list(User.objects.using('other').values_list('username',flat=True)),
                          [u'bob'])
        self.assertEqual(list(UserProfile.objects.using('other').values_list('flavor',flat=True)),
                           [u'crunchy frog'])

        # When saved (no using required), new objects goes to 'other'
        charlie.save()
        bob_profile.save()
        new_bob_profile.save()
        self.assertEqual(list(User.objects.using('default').values_list('username',flat=True)),
                          [u'alice'])
        self.assertEqual(list(User.objects.using('other').values_list('username',flat=True)),
                          [u'bob', u'charlie'])
        self.assertEqual(list(UserProfile.objects.using('default').values_list('flavor',flat=True)),
                           [u'chocolate'])
        self.assertEqual(list(UserProfile.objects.using('other').values_list('flavor',flat=True)),
                           [u'crunchy frog', u'spring surprise'])

        # This also works if you assign the O2O relation in the constructor
        denise = User.objects.db_manager('other').create_user('denise','denise@example.com')
        denise_profile = UserProfile(flavor="tofu", user=denise)

        self.assertEqual(denise_profile._state.db, 'other')
        # ... but it isn't saved yet
        self.assertEqual(list(UserProfile.objects.using('default').values_list('flavor',flat=True)),
                           [u'chocolate'])
        self.assertEqual(list(UserProfile.objects.using('other').values_list('flavor',flat=True)),
                           [u'crunchy frog', u'spring surprise'])

        # When saved, the new profile goes to 'other'
        denise_profile.save()
        self.assertEqual(list(UserProfile.objects.using('default').values_list('flavor',flat=True)),
                           [u'chocolate'])
        self.assertEqual(list(UserProfile.objects.using('other').values_list('flavor',flat=True)),
                           [u'crunchy frog', u'spring surprise', u'tofu'])
Operations that involve sharing FK objects across databases raise an error
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.4/tests/regressiontests/multiple_database/tests.py/QueryTestCase.test_o2o_cross_database_protection