[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

1 files changed, 3 insertions, 0 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 043eac908b..abe882eaeb 100644
--- a/django/db/models/query.py
+++ b/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]