Fixed #18177 -- Cached known related instances.

This was recently fixed for one-to-one relations; this patch adds
support for foreign keys. Thanks kaiser.yann for the report and
the initial version of the patch.

django/db/models/fields/related.py

@@ -237,13 +237,18 @@ class SingleRelatedObjectDescriptor(object):
         return self.related.model._base_manager.using(db)
 
     def get_prefetch_query_set(self, instances):
-        vals = set(instance._get_pk_val() for instance in instances)
-        params = {'%s__pk__in' % self.related.field.name: vals}
-        return (self.get_query_set(instance=instances[0]).filter(**params),
-                attrgetter(self.related.field.attname),
-                lambda obj: obj._get_pk_val(),
-                True,
-                self.cache_name)
+        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)
+        params = {'%s__pk__in' % self.related.field.name: instances_dict.keys()}
+        qs = self.get_query_set(instance=instances[0]).filter(**params)
+        # 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:
+            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
 
     def __get__(self, instance, instance_type=None):
         if instance is None:
@@ -324,17 +329,23 @@ class ReverseSingleRelatedObjectDescriptor(object):
             return QuerySet(self.field.rel.to).using(db)
 
     def get_prefetch_query_set(self, instances):
-        vals = set(getattr(instance, self.field.attname) for instance in instances)
+        rel_obj_attr = attrgetter(self.field.rel.field_name)
+        instance_attr = attrgetter(self.field.attname)
+        instances_dict = dict((instance_attr(inst), inst) for inst in instances)
         other_field = self.field.rel.get_related_field()
         if other_field.rel:
-            params = {'%s__pk__in' % self.field.rel.field_name: vals}
+            params = {'%s__pk__in' % self.field.rel.field_name: instances_dict.keys()}
         else:
-            params = {'%s__in' % self.field.rel.field_name: vals}
-        return (self.get_query_set(instance=instances[0]).filter(**params),
-                attrgetter(self.field.rel.field_name),
-                attrgetter(self.field.attname),
-                True,
-                self.cache_name)
+            params = {'%s__in' % self.field.rel.field_name: instances_dict.keys()}
+        qs = self.get_query_set(instance=instances[0]).filter(**params)
+        # 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:
+                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
 
     def __get__(self, instance, instance_type=None):
         if instance is None:
@@ -467,18 +478,24 @@ class ForeignRelatedObjectsDescriptor(object):
                     return self.instance._prefetched_objects_cache[rel_field.related_query_name()]
                 except (AttributeError, KeyError):
                     db = self._db or router.db_for_read(self.model, instance=self.instance)
-                    return super(RelatedManager, self).get_query_set().using(db).filter(**self.core_filters)
+                    qs = super(RelatedManager, self).get_query_set().using(db).filter(**self.core_filters)
+                    qs._known_related_object = (rel_field.name, self.instance)
+                    return qs
 
             def get_prefetch_query_set(self, instances):
+                rel_obj_attr = attrgetter(rel_field.get_attname())
+                instance_attr = attrgetter(attname)
+                instances_dict = dict((instance_attr(inst), inst) for inst in instances)
                 db = self._db or router.db_for_read(self.model, instance=instances[0])
-                query = {'%s__%s__in' % (rel_field.name, attname):
-                             set(getattr(obj, attname) for obj in instances)}
+                query = {'%s__%s__in' % (rel_field.name, attname): instances_dict.keys()}
                 qs = super(RelatedManager, self).get_query_set().using(db).filter(**query)
-                return (qs,
-                        attrgetter(rel_field.get_attname()),
-                        attrgetter(attname),
-                        False,
-                        rel_field.related_query_name())
+                # Since we just bypassed this class' get_query_set(), we must manage
+                # the reverse relation manually.
+                for rel_obj in qs:
+                    instance = instances_dict[rel_obj_attr(rel_obj)]
+                    setattr(rel_obj, rel_field.name, instance)
+                cache_name = rel_field.related_query_name()
+                return qs, rel_obj_attr, instance_attr, False, cache_name
 
             def add(self, *objs):
                 for obj in objs:

django/db/models/query.py

@@ -41,6 +41,7 @@ class QuerySet(object):
         self._for_write = False
         self._prefetch_related_lookups = []
         self._prefetch_done = False
+        self._known_related_object = None       # (attname, rel_obj)
 
     ########################
     # PYTHON MAGIC METHODS #
@@ -282,9 +283,10 @@ class QuerySet(object):
                     init_list.append(field.attname)
             model_cls = deferred_class_factory(self.model, skip)
 
-        # Cache db and model outside the loop
+        # Cache db, model and known_related_object outside the loop
         db = self.db
         model = self.model
+        kro_attname, kro_instance = self._known_related_object or (None, None)
         compiler = self.query.get_compiler(using=db)
         if fill_cache:
             klass_info = get_klass_info(model, max_depth=max_depth,
@@ -294,12 +296,12 @@ class QuerySet(object):
                 obj, _ = get_cached_row(row, index_start, db, klass_info,
                                         offset=len(aggregate_select))
             else:
-                if skip:
+                # Omit aggregates in object creation.
                 row_data = row[index_start:aggregate_start]
+                if skip:
                     obj = model_cls(**dict(zip(init_list, row_data)))
                 else:
-                    # Omit aggregates in object creation.
-                    obj = model(*row[index_start:aggregate_start])
+                    obj = model(*row_data)
 
                 # Store the source database of the object
                 obj._state.db = db
@@ -313,7 +315,11 @@ class QuerySet(object):
             # Add the aggregates to the model
             if aggregate_select:
                 for i, aggregate in enumerate(aggregate_select):
-                    setattr(obj, aggregate, row[i+aggregate_start])
+                    setattr(obj, aggregate, row[i + aggregate_start])
+
+            # Add the known related object to the model, if there is one
+            if kro_instance:
+                setattr(obj, kro_attname, kro_instance)
 
             yield obj
 
@@ -864,6 +870,7 @@ class QuerySet(object):
         c = klass(model=self.model, query=query, using=self._db)
         c._for_write = self._for_write
         c._prefetch_related_lookups = self._prefetch_related_lookups[:]
+        c._known_related_object = self._known_related_object
         c.__dict__.update(kwargs)
         if setup and hasattr(c, '_setup_query'):
             c._setup_query()
@@ -1781,9 +1788,7 @@ def prefetch_one_level(instances, prefetcher, attname):
     rel_obj_cache = {}
     for rel_obj in all_related_objects:
         rel_attr_val = rel_obj_attr(rel_obj)
-        if rel_attr_val not in rel_obj_cache:
-            rel_obj_cache[rel_attr_val] = []
-        rel_obj_cache[rel_attr_val].append(rel_obj)
+        rel_obj_cache.setdefault(rel_attr_val, []).append(rel_obj)
 
     for obj in instances:
         instance_attr_val = instance_attr(obj)

docs/releases/1.5.txt

@@ -44,6 +44,24 @@ reasons or when trying to avoid overwriting concurrent changes.
 See the :meth:`Model.save() <django.db.models.Model.save()>` documentation for
 more details.
 
+Caching of related model instances
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When traversing relations, the ORM will avoid re-fetching objects that were
+previously loaded. For example, with the tutorial's models::
+
+    >>> first_poll = Poll.objects.all()[0]
+    >>> first_choice = first_poll.choice_set.all()[0]
+    >>> first_choice.poll is first_poll
+    True
+
+In Django 1.5, the third line no longer triggers a new SQL query to fetch
+``first_choice.poll``; it was set when by the second line.
+
+For one-to-one relationships, both sides can be cached. For many-to-one
+relationships, only the single side of the relationship can be cached. This
+is particularly helpful in combination with ``prefetch_related``.
+
 Minor features
 ~~~~~~~~~~~~~~
 

tests/modeltests/known_related_objects/fixtures/tournament.json

@@ -0,0 +1,65 @@
+[
+    {
+        "pk": 1,
+        "model": "known_related_objects.tournament",
+        "fields": {
+            "name": "Tourney 1"
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.tournament",
+        "fields": {
+            "name": "Tourney 2"
+            }
+        },
+    {
+        "pk": 1,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 1,
+            "name": "T1 Pool 1"
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 1,
+            "name": "T1 Pool 2"
+            }
+        },
+    {
+        "pk": 3,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 2,
+            "name": "T2 Pool 1"
+            }
+        },
+    {
+        "pk": 4,
+        "model": "known_related_objects.pool",
+        "fields": {
+            "tournament": 2,
+            "name": "T2 Pool 2"
+            }
+        },
+    {
+        "pk": 1,
+        "model": "known_related_objects.poolstyle",
+        "fields": {
+            "name": "T1 Pool 2 Style",
+            "pool": 2
+            }
+        },
+    {
+        "pk": 2,
+        "model": "known_related_objects.poolstyle",
+        "fields": {
+            "name": "T2 Pool 1 Style",
+            "pool": 3
+            }
+        }
+]
+

tests/modeltests/known_related_objects/models.py

@@ -0,0 +1,19 @@
+"""
+Existing related object instance caching.
+
+Test that queries are not redone when going back through known relations.
+"""
+
+from django.db import models
+
+class Tournament(models.Model):
+    name = models.CharField(max_length=30)
+
+class Pool(models.Model):
+    name = models.CharField(max_length=30)
+    tournament = models.ForeignKey(Tournament)
+
+class PoolStyle(models.Model):
+    name = models.CharField(max_length=30)
+    pool = models.OneToOneField(Pool)
+

tests/modeltests/known_related_objects/tests.py

@@ -0,0 +1,88 @@
+from __future__ import absolute_import
+
+from django.test import TestCase
+
+from .models import Tournament, Pool, PoolStyle
+
+class ExistingRelatedInstancesTests(TestCase):
+    fixtures = ['tournament.json']
+
+    def test_foreign_key(self):
+        with self.assertNumQueries(2):
+            tournament = Tournament.objects.get(pk=1)
+            pool = tournament.pool_set.all()[0]
+            self.assertIs(tournament, pool.tournament)
+
+    def test_foreign_key_prefetch_related(self):
+        with self.assertNumQueries(2):
+            tournament = (Tournament.objects.prefetch_related('pool_set').get(pk=1))
+            pool = tournament.pool_set.all()[0]
+            self.assertIs(tournament, pool.tournament)
+
+    def test_foreign_key_multiple_prefetch(self):
+        with self.assertNumQueries(2):
+            tournaments = list(Tournament.objects.prefetch_related('pool_set'))
+            pool1 = tournaments[0].pool_set.all()[0]
+            self.assertIs(tournaments[0], pool1.tournament)
+            pool2 = tournaments[1].pool_set.all()[0]
+            self.assertIs(tournaments[1], pool2.tournament)
+
+    def test_one_to_one(self):
+        with self.assertNumQueries(2):
+            style = PoolStyle.objects.get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_select_related(self):
+        with self.assertNumQueries(1):
+            style = PoolStyle.objects.select_related('pool').get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_multi_select_related(self):
+        with self.assertNumQueries(1):
+            poolstyles = list(PoolStyle.objects.select_related('pool'))
+            self.assertIs(poolstyles[0], poolstyles[0].pool.poolstyle)
+            self.assertIs(poolstyles[1], poolstyles[1].pool.poolstyle)
+
+    def test_one_to_one_prefetch_related(self):
+        with self.assertNumQueries(2):
+            style = PoolStyle.objects.prefetch_related('pool').get(pk=1)
+            pool = style.pool
+            self.assertIs(style, pool.poolstyle)
+
+    def test_one_to_one_multi_prefetch_related(self):
+        with self.assertNumQueries(2):
+            poolstyles = list(PoolStyle.objects.prefetch_related('pool'))
+            self.assertIs(poolstyles[0], poolstyles[0].pool.poolstyle)
+            self.assertIs(poolstyles[1], poolstyles[1].pool.poolstyle)
+
+    def test_reverse_one_to_one(self):
+        with self.assertNumQueries(2):
+            pool = Pool.objects.get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_select_related(self):
+        with self.assertNumQueries(1):
+            pool = Pool.objects.select_related('poolstyle').get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_prefetch_related(self):
+        with self.assertNumQueries(2):
+            pool = Pool.objects.prefetch_related('poolstyle').get(pk=2)
+            style = pool.poolstyle
+            self.assertIs(pool, style.pool)
+
+    def test_reverse_one_to_one_multi_select_related(self):
+        with self.assertNumQueries(1):
+            pools = list(Pool.objects.select_related('poolstyle'))
+            self.assertIs(pools[1], pools[1].poolstyle.pool)
+            self.assertIs(pools[2], pools[2].poolstyle.pool)
+
+    def test_reverse_one_to_one_multi_prefetch_related(self):
+        with self.assertNumQueries(2):
+            pools = list(Pool.objects.prefetch_related('poolstyle'))
+            self.assertIs(pools[1], pools[1].poolstyle.pool)
+            self.assertIs(pools[2], pools[2].poolstyle.pool)