[1.10.x] Fixed #26676 -- Prevented prefetching to_attr from caching its result in through attr.

Thanks Ursidours for the report.

Backport of 53a5fb3cc0 from master

django/db/models/query.py

@@ -1451,7 +1451,8 @@ def prefetch_related_objects(model_instances, *related_lookups):
             # We assume that objects retrieved are homogeneous (which is the premise
             # of prefetch_related), so what applies to first object applies to all.
             first_obj = obj_list[0]
-            prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(first_obj, through_attr)
+            to_attr = lookup.get_current_to_attr(level)[0]
+            prefetcher, descriptor, attr_found, is_fetched = get_prefetcher(first_obj, through_attr, to_attr)
 
             if not attr_found:
                 raise AttributeError("Cannot find '%s' on %s object, '%s' is an invalid "
@@ -1504,9 +1505,9 @@ def prefetch_related_objects(model_instances, *related_lookups):
                 obj_list = new_obj_list
 
 
-def get_prefetcher(instance, attr):
+def get_prefetcher(instance, through_attr, to_attr):
     """
-    For the attribute 'attr' on the given instance, finds
+    For the attribute 'through_attr' on the given instance, finds
     an object that has a get_prefetch_queryset().
     Returns a 4 tuple containing:
     (the object with get_prefetch_queryset (or None),
@@ -1520,9 +1521,9 @@ def get_prefetcher(instance, attr):
     # For singly related objects, we have to avoid getting the attribute
     # from the object, as this will trigger the query. So we first try
     # on the class, in order to get the descriptor object.
-    rel_obj_descriptor = getattr(instance.__class__, attr, None)
+    rel_obj_descriptor = getattr(instance.__class__, through_attr, None)
     if rel_obj_descriptor is None:
-        attr_found = hasattr(instance, attr)
+        attr_found = hasattr(instance, through_attr)
     else:
         attr_found = True
         if rel_obj_descriptor:
@@ -1536,10 +1537,13 @@ def get_prefetcher(instance, attr):
                 # descriptor doesn't support prefetching, so we go ahead and get
                 # the attribute on the instance rather than the class to
                 # support many related managers
-                rel_obj = getattr(instance, attr)
+                rel_obj = getattr(instance, through_attr)
                 if hasattr(rel_obj, 'get_prefetch_queryset'):
                     prefetcher = rel_obj
-                is_fetched = attr in instance._prefetched_objects_cache
+                if through_attr != to_attr:
+                    is_fetched = hasattr(instance, to_attr)
+                else:
+                    is_fetched = through_attr in instance._prefetched_objects_cache
     return prefetcher, rel_obj_descriptor, attr_found, is_fetched
 
 
@@ -1619,7 +1623,6 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
         else:
             if as_attr:
                 setattr(obj, to_attr, vals)
-                obj._prefetched_objects_cache[cache_name] = vals
             else:
                 manager = getattr(obj, to_attr)
                 if leaf and lookup.queryset is not None:

tests/prefetch_related/tests.py

@@ -5,7 +5,7 @@ import warnings
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist
 from django.db import connection
-from django.db.models import Prefetch
+from django.db.models import Prefetch, QuerySet
 from django.db.models.query import get_prefetcher
 from django.test import TestCase, override_settings
 from django.test.utils import CaptureQueriesContext
@@ -737,6 +737,12 @@ class CustomPrefetchTests(TestCase):
         with self.assertRaisesMessage(ValueError, 'Prefetch querysets cannot use values().'):
             Prefetch('houses', House.objects.values('pk'))
 
+    def test_to_attr_doesnt_cache_through_attr_as_list(self):
+        house = House.objects.prefetch_related(
+            Prefetch('rooms', queryset=Room.objects.all(), to_attr='to_rooms'),
+        ).get(pk=self.house3.pk)
+        self.assertIsInstance(house.rooms.all(), QuerySet)
+
 
 class DefaultManagerTests(TestCase):
 
@@ -1268,7 +1274,7 @@ class Ticket21760Tests(TestCase):
             house.save()
 
     def test_bug(self):
-        prefetcher = get_prefetcher(self.rooms[0], 'house')[0]
+        prefetcher = get_prefetcher(self.rooms[0], 'house', 'house')[0]
         queryset = prefetcher.get_prefetch_queryset(list(Room.objects.all()))[0]
         self.assertNotIn(' JOIN ', force_text(queryset.query))