Allowed custom querysets when prefetching single valued relations

The original patch for custom prefetches didn't allow usage of custom
queryset for single valued relations (along ForeignKey or OneToOneKey).
Allowing these enables calling performance oriented queryset methods like
select_related or defer/only.

Thanks @akaariai and @timgraham for the reviews. Refs #17001.

django/db/models/fields/related.py

@@ -375,24 +375,29 @@ class SingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjectDescri
         return hasattr(instance, self.cache_name)
 
     def get_queryset(self, **hints):
+        # Gotcha: we return a `Manager` instance (i.e. not a `QuerySet`)!
         return self.related.model._base_manager.db_manager(hints=hints)
 
     def get_prefetch_queryset(self, instances, queryset=None):
-        if queryset is not None:
+        if queryset is None:
-            raise ValueError("Custom queryset can't be used for this lookup.")
+            # Despite its name `get_queryset()` returns an instance of
+            # `Manager`, therefore we call `all()` to normalize to `QuerySet`.
+            queryset = self.get_queryset().all()
+        queryset._add_hints(instance=instances[0])
 
         rel_obj_attr = attrgetter(self.related.field.attname)
         instance_attr = lambda obj: obj._get_pk_val()
         instances_dict = dict((instance_attr(inst), inst) for inst in instances)
         query = {'%s__in' % self.related.field.name: instances}
-        qs = self.get_queryset(instance=instances[0]).filter(**query)
+        queryset = queryset.filter(**query)
+
         # Since we're going to assign directly in the cache,
         # we must manage the reverse relation cache manually.
         rel_obj_cache_name = self.related.field.get_cache_name()
-        for rel_obj in qs:
+        for rel_obj in queryset:
             instance = instances_dict[rel_obj_attr(rel_obj)]
             setattr(rel_obj, rel_obj_cache_name, instance)
-        return qs, rel_obj_attr, instance_attr, True, self.cache_name
+        return queryset, rel_obj_attr, instance_attr, True, self.cache_name
 
     def __get__(self, instance, instance_type=None):
         if instance is None:
@@ -503,13 +508,17 @@ class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjec
         # If the related manager indicates that it should be used for
         # related fields, respect that.
         if getattr(rel_mgr, 'use_for_related_fields', False):
+            # Gotcha: we return a `Manager` instance (i.e. not a `QuerySet`)!
             return rel_mgr
         else:
             return QuerySet(self.field.rel.to, hints=hints)
 
     def get_prefetch_queryset(self, instances, queryset=None):
-        if queryset is not None:
+        if queryset is None:
-            raise ValueError("Custom queryset can't be used for this lookup.")
+            # Despite its name `get_queryset()` may return an instance of
+            # `Manager`, therefore we call `all()` to normalize to `QuerySet`.
+            queryset = self.get_queryset().all()
+        queryset._add_hints(instance=instances[0])
 
         rel_obj_attr = self.field.get_foreign_related_value
         instance_attr = self.field.get_local_related_value
@@ -524,16 +533,16 @@ class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjec
             query = {'%s__in' % related_field.name: set(instance_attr(inst)[0] for inst in instances)}
         else:
             query = {'%s__in' % self.field.related_query_name(): instances}
+        queryset = queryset.filter(**query)
 
-        qs = self.get_queryset(instance=instances[0]).filter(**query)
         # Since we're going to assign directly in the cache,
         # we must manage the reverse relation cache manually.
         if not self.field.rel.multiple:
             rel_obj_cache_name = self.field.related.get_cache_name()
-            for rel_obj in qs:
+            for rel_obj in queryset:
                 instance = instances_dict[rel_obj_attr(rel_obj)]
                 setattr(rel_obj, rel_obj_cache_name, instance)
-        return qs, rel_obj_attr, instance_attr, True, self.cache_name
+        return queryset, rel_obj_attr, instance_attr, True, self.cache_name
 
     def __get__(self, instance, instance_type=None):
         if instance is None:

django/db/models/query.py

@@ -1667,8 +1667,8 @@ class Prefetch(object):
     def get_current_to_attr(self, level):
         parts = self.prefetch_to.split(LOOKUP_SEP)
         to_attr = parts[level]
-        to_list = self.to_attr and level == len(parts) - 1
+        as_attr = self.to_attr and level == len(parts) - 1
-        return to_attr, to_list
+        return to_attr, as_attr
 
     def get_current_queryset(self, level):
         if self.get_current_prefetch_to(level) == self.prefetch_to:
@@ -1913,12 +1913,13 @@ def prefetch_one_level(instances, prefetcher, lookup, level):
     for obj in instances:
         instance_attr_val = instance_attr(obj)
         vals = rel_obj_cache.get(instance_attr_val, [])
+        to_attr, as_attr = lookup.get_current_to_attr(level)
         if single:
-            # Need to assign to single cache on instance
+            val = vals[0] if vals else None
-            setattr(obj, cache_name, vals[0] if vals else None)
+            to_attr = to_attr if as_attr else cache_name
+            setattr(obj, to_attr, val)
         else:
-            to_attr, to_list = lookup.get_current_to_attr(level)
+            if as_attr:
-            if to_list:
                 setattr(obj, to_attr, vals)
             else:
                 # Cache in the QuerySet.all().

docs/ref/models/querysets.txt

@@ -989,6 +989,24 @@ less ambiguous than storing a filtered result in the related manager's cache:
     ...     Prefetch('pizzas', queryset=queryset))
     >>> vegetarian_pizzas = restaurants[0].pizzas.all()
 
+Custom prefetching also works with single related relations like
+forward ``ForeignKey`` or ``OneToOneField``. Generally you'll want to use
+:meth:`select_related()` for these relations, but there are a number of cases
+where prefetching with a custom ``QuerySet`` is useful:
+
+* You want to use a ``QuerySet`` that performs further prefetching
+  on related models.
+
+* You want to prefetch only a subset of the related objects.
+
+* You want to use performance optimization techniques like
+  :meth:`deferred fields <defer()>`:
+
+    >>> queryset = Pizza.objects.only('name')
+    >>>
+    >>> restaurants = Restaurant.objects.prefetch_related(
+    ...     Prefetch('best_pizza', queryset=queryset))
+
 .. note::
 
     The ordering of lookups matters.

tests/prefetch_related/models.py

@@ -170,8 +170,10 @@ class Comment(models.Model):
 ## Models for lookup ordering tests
 
 class House(models.Model):
+    name = models.CharField(max_length=50)
     address = models.CharField(max_length=255)
     owner = models.ForeignKey('Person', null=True)
+    main_room = models.OneToOneField('Room', related_name='main_room_of', null=True)
 
     class Meta:
         ordering = ['id']

tests/prefetch_related/tests.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from django.core.exceptions import ObjectDoesNotExist
 from django.contrib.contenttypes.models import ContentType
 from django.db import connection
 from django.db.models import Prefetch
@@ -228,17 +229,22 @@ class CustomPrefetchTests(TestCase):
             for part in path:
                 if not part:
                     continue
-                rel_objs.extend(cls.traverse_qs(getattr(obj, part[0]), [part[1:]]))
+                try:
+                    related = getattr(obj, part[0])
+                except ObjectDoesNotExist:
+                    continue
+                if related is not None:
+                    rel_objs.extend(cls.traverse_qs(related, [part[1:]]))
             ret_val.append((obj, rel_objs))
         return ret_val
 
     def setUp(self):
         self.person1 = Person.objects.create(name="Joe")
         self.person2 = Person.objects.create(name="Mary")
-        self.house1 = House.objects.create(address="123 Main St", owner=self.person1)
+        self.house1 = House.objects.create(name='House 1', address="123 Main St", owner=self.person1)
-        self.house2 = House.objects.create(address="45 Side St", owner=self.person1)
+        self.house2 = House.objects.create(name='House 2', address="45 Side St", owner=self.person1)
-        self.house3 = House.objects.create(address="6 Downing St", owner=self.person2)
+        self.house3 = House.objects.create(name='House 3', address="6 Downing St", owner=self.person2)
-        self.house4 = House.objects.create(address="7 Regents St", owner=self.person2)
+        self.house4 = House.objects.create(name='house 4', address="7 Regents St", owner=self.person2)
         self.room1_1 = Room.objects.create(name="Dining room", house=self.house1)
         self.room1_2 = Room.objects.create(name="Lounge", house=self.house1)
         self.room1_3 = Room.objects.create(name="Kitchen", house=self.house1)
@@ -253,6 +259,14 @@ class CustomPrefetchTests(TestCase):
         self.room4_3 = Room.objects.create(name="Kitchen", house=self.house4)
         self.person1.houses.add(self.house1, self.house2)
         self.person2.houses.add(self.house3, self.house4)
+        self.house1.main_room = self.room1_1
+        self.house1.save()
+        self.house2.main_room = self.room2_1
+        self.house2.save()
+        self.house3.main_room = self.room3_1
+        self.house3.save()
+        self.house4.main_room = self.room4_1
+        self.house4.save()
 
     def test_traverse_qs(self):
         qs = Person.objects.prefetch_related('houses')
@@ -262,16 +276,17 @@ class CustomPrefetchTests(TestCase):
         self.assertEqual(related_objs_normal, (related_objs_from_traverse,))
 
     def test_ambiguous(self):
-        # Ambiguous.
+        # Ambiguous: Lookup was already seen with a different queryset.
         with self.assertRaises(ValueError):
             self.traverse_qs(
                 Person.objects.prefetch_related('houses__rooms', Prefetch('houses', queryset=House.objects.all())),
                 [['houses', 'rooms']]
             )
 
+        # Ambiguous: Lookup houses_lst doesn't yet exist when performing houses_lst__rooms.
         with self.assertRaises(AttributeError):
             self.traverse_qs(
-                Person.objects.prefetch_related('houses_list__rooms', Prefetch('houses', queryset=House.objects.all(), to_attr='houses_lst')),
+                Person.objects.prefetch_related('houses_lst__rooms', Prefetch('houses', queryset=House.objects.all(), to_attr='houses_lst')),
                 [['houses', 'rooms']]
             )
 
@@ -491,6 +506,50 @@ class CustomPrefetchTests(TestCase):
         self.assertEqual(lst2[0].houses_lst[0].rooms_lst[1], self.room1_2)
         self.assertEqual(len(lst2[1].houses_lst), 0)
 
+        # Test ReverseSingleRelatedObjectDescriptor.
+        houses = House.objects.select_related('owner')
+        with self.assertNumQueries(6):
+            rooms = Room.objects.all().prefetch_related('house')
+            lst1 = self.traverse_qs(rooms, [['house', 'owner']])
+        with self.assertNumQueries(2):
+            rooms = Room.objects.all().prefetch_related(Prefetch('house', queryset=houses.all()))
+            lst2 = self.traverse_qs(rooms, [['house', 'owner']])
+        self.assertEqual(lst1, lst2)
+        with self.assertNumQueries(2):
+            houses = House.objects.select_related('owner')
+            rooms = Room.objects.all().prefetch_related(Prefetch('house', queryset=houses.all(), to_attr='house_attr'))
+            lst2 = self.traverse_qs(rooms, [['house_attr', 'owner']])
+        self.assertEqual(lst1, lst2)
+        room = Room.objects.all().prefetch_related(Prefetch('house', queryset=houses.filter(address='DoesNotExist'))).first()
+        with self.assertRaises(ObjectDoesNotExist):
+            getattr(room, 'house')
+        room = Room.objects.all().prefetch_related(Prefetch('house', queryset=houses.filter(address='DoesNotExist'), to_attr='house_attr')).first()
+        self.assertIsNone(room.house_attr)
+        rooms = Room.objects.all().prefetch_related(Prefetch('house', queryset=House.objects.only('name')))
+        with self.assertNumQueries(2):
+            getattr(rooms.first().house, 'name')
+        with self.assertNumQueries(3):
+            getattr(rooms.first().house, 'address')
+
+        # Test SingleRelatedObjectDescriptor.
+        houses = House.objects.select_related('owner')
+        with self.assertNumQueries(6):
+            rooms = Room.objects.all().prefetch_related('main_room_of')
+            lst1 = self.traverse_qs(rooms, [['main_room_of', 'owner']])
+        with self.assertNumQueries(2):
+            rooms = Room.objects.all().prefetch_related(Prefetch('main_room_of', queryset=houses.all()))
+            lst2 = self.traverse_qs(rooms, [['main_room_of', 'owner']])
+        self.assertEqual(lst1, lst2)
+        with self.assertNumQueries(2):
+            rooms = list(Room.objects.all().prefetch_related(Prefetch('main_room_of', queryset=houses.all(), to_attr='main_room_of_attr')))
+            lst2 = self.traverse_qs(rooms, [['main_room_of_attr', 'owner']])
+        self.assertEqual(lst1, lst2)
+        room = Room.objects.filter(main_room_of__isnull=False).prefetch_related(Prefetch('main_room_of', queryset=houses.filter(address='DoesNotExist'))).first()
+        with self.assertRaises(ObjectDoesNotExist):
+            getattr(room, 'main_room_of')
+        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)
+
 
 class DefaultManagerTests(TestCase):