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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.
This commit is contained in:
Loic Bistuer 2014-01-17 01:24:39 +07:00 committed by Anssi Kääriäinen
parent c8d61fa109
commit 7bbb6958dc
5 changed files with 112 additions and 23 deletions
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29
django/db/models/fields/related.py
View File

@ -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:
raise ValueError("Custom queryset can't be used for this lookup.")
if queryset is None:
# 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:
raise ValueError("Custom queryset can't be used for this lookup.")
if queryset is None:
# 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:

13
django/db/models/query.py
View File

@ -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
return to_attr, to_list
as_attr = self.to_attr and level == len(parts) - 1
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
setattr(obj, cache_name, vals[0] if vals else None)
val = 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 to_list:
if as_attr:
setattr(obj, to_attr, vals)
else:
# Cache in the QuerySet.all().

18
docs/ref/models/querysets.txt
View File

@ -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.

2
tests/prefetch_related/models.py
View File

@ -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']

73
tests/prefetch_related/tests.py
View File

@ -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.house2 = House.objects.create(address="45 Side St", owner=self.person1)
self.house3 = House.objects.create(address="6 Downing St", owner=self.person2)
self.house4 = House.objects.create(address="7 Regents St", owner=self.person2)
self.house1 = House.objects.create(name='House 1', address="123 Main St", owner=self.person1)
self.house2 = House.objects.create(name='House 2', address="45 Side St", owner=self.person1)
self.house3 = House.objects.create(name='House 3', address="6 Downing 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):