[1.7.x] Fixed #22508 -- Avoided overwriting select_related.

Previously, known related objects overwrote related objects loaded
though select_related. This could cancel the effect of select_related
when it was used over more than one level.

Thanks boxm for the bug report and timo for bisecting the regression.

Backport of f574220f from master

django/db/models/query.py

@@ -291,6 +291,9 @@ class QuerySet(object):
             # Add the known related objects to the model, if there are any
             if self._known_related_objects:
                 for field, rel_objs in self._known_related_objects.items():
+                    # Avoid overwriting objects loaded e.g. by select_related
+                    if hasattr(obj, field.get_cache_name()):
+                        continue
                     pk = getattr(obj, field.get_attname())
                     try:
                         rel_obj = rel_objs[pk]

tests/select_related_regress/tests.py

@@ -188,3 +188,13 @@ class SelectRelatedRegressTests(TestCase):
             self.assertEqual(qs_c.c_b.lots_of_text, 'lots_of_text_b')
             self.assertEqual(qs_c.c_a.name, 'a')
             self.assertEqual(qs_c.c_b.name, 'b')
+
+    def test_regression_22508(self):
+        building = Building.objects.create(name='101')
+        device = Device.objects.create(name="router", building=building)
+        Port.objects.create(port_number='1', device=device)
+
+        device = Device.objects.get()
+        port = device.port_set.select_related('device__building').get()
+        with self.assertNumQueries(0):
+            port.device.building