Fixed #24873 -- Prevented nested Prefetch objects from being overwritten.

django/db/models/query.py

@@ -1538,7 +1538,12 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
     # contains some prefetch_related lookups. We don't want to trigger the
     # prefetch_related functionality by evaluating the query. Rather, we need
     # to merge in the prefetch_related lookups.
-    additional_lookups = getattr(rel_qs, '_prefetch_related_lookups', [])
+    # Copy the lookups in case it is a Prefetch object which could be reused
+    # later (happens in nested prefetch_related).
+    additional_lookups = [
+        copy.copy(additional_lookup) for additional_lookup
+        in getattr(rel_qs, '_prefetch_related_lookups', [])
+    ]
     if additional_lookups:
         # Don't need to clone because the manager should have given us a fresh
         # instance, so we access an internal instead of using public interface

tests/prefetch_related/tests.py

@@ -626,6 +626,17 @@ class CustomPrefetchTests(TestCase):
         room = Room.objects.filter(main_room_of__isnull=False).prefetch_related(Prefetch('main_room_of', queryset=houses.filter(address='DoesNotExist'), to_attr='main_room_of_attr')).first()
         self.assertIsNone(room.main_room_of_attr)
 
+    def test_nested_prefetch_related_are_not_overwritten(self):
+        # Regression test for #24873
+        houses_2 = House.objects.prefetch_related(Prefetch('rooms'))
+        persons = Person.objects.prefetch_related(Prefetch('houses', queryset=houses_2))
+        houses = House.objects.prefetch_related(Prefetch('occupants', queryset=persons))
+        list(houses)  # queryset must be evaluated once to reproduce the bug.
+        self.assertEqual(
+            houses.all()[0].occupants.all()[0].houses.all()[1].rooms.all()[0],
+            self.room2_1
+        )
+
 
 class DefaultManagerTests(TestCase):