Hyperlinked PK optimization.

rest_framework/relations.py

@@ -84,9 +84,20 @@ def get_queryset(self):
             queryset = queryset.all()
         return queryset
 
-    def get_iterable(self, instance, source_attrs):
-        relationship = get_attribute(instance, source_attrs)
-        return relationship.all() if (hasattr(relationship, 'all')) else relationship
+    def use_pk_only_optimization(self):
+        return False
+
+    def get_attribute(self, instance):
+        if self.use_pk_only_optimization() and self.source_attrs:
+            # Optimized case, return a mock object only containing the pk attribute.
+            try:
+                instance = get_attribute(instance, self.source_attrs[:-1])
+                return PKOnlyObject(pk=instance.serializable_value(self.source_attrs[-1]))
+            except AttributeError:
+                pass
+
+        # Standard case, return the object instance.
+        return get_attribute(instance, self.source_attrs)
 
     @property
     def choices(self):
@@ -120,6 +131,9 @@ class PrimaryKeyRelatedField(RelatedField):
         'incorrect_type': _('Incorrect type. Expected pk value, received {data_type}.'),
     }
 
+    def use_pk_only_optimization(self):
+        return True
+
     def to_internal_value(self, data):
         try:
             return self.get_queryset().get(pk=data)
@@ -128,32 +142,6 @@ def to_internal_value(self, data):
         except (TypeError, ValueError):
             self.fail('incorrect_type', data_type=type(data).__name__)
 
-    def get_attribute(self, instance):
-        # We customize `get_attribute` here for performance reasons.
-        # For relationships the instance will already have the pk of
-        # the related object. We return this directly instead of returning the
-        # object itself, which would require a database lookup.
-        try:
-            instance = get_attribute(instance, self.source_attrs[:-1])
-            return PKOnlyObject(pk=instance.serializable_value(self.source_attrs[-1]))
-        except AttributeError:
-            return get_attribute(instance, self.source_attrs)
-
-    def get_iterable(self, instance, source_attrs):
-        # For consistency with `get_attribute` we're using `serializable_value()`
-        # here. Typically there won't be any difference, but some custom field
-        # types might return a non-primitive value for the pk otherwise.
-        #
-        # We could try to get smart with `values_list('pk', flat=True)`, which
-        # would be better in some case, but would actually end up with *more*
-        # queries if the developer is using `prefetch_related` across the
-        # relationship.
-        relationship = super(PrimaryKeyRelatedField, self).get_iterable(instance, source_attrs)
-        return [
-            PKOnlyObject(pk=item.serializable_value('pk'))
-            for item in relationship
-        ]
-
     def to_representation(self, value):
         return value.pk
 
@@ -184,6 +172,9 @@ def __init__(self, view_name=None, **kwargs):
 
         super(HyperlinkedRelatedField, self).__init__(**kwargs)
 
+    def use_pk_only_optimization(self):
+        return self.lookup_field == 'pk'
+
     def get_object(self, view_name, view_args, view_kwargs):
         """
         Return the object corresponding to a matched URL.
@@ -285,6 +276,11 @@ def __init__(self, view_name=None, **kwargs):
         kwargs['source'] = '*'
         super(HyperlinkedIdentityField, self).__init__(view_name, **kwargs)
 
+    def use_pk_only_optimization(self):
+        # We have the complete object instance already. We don't need
+        # to run the 'only get the pk for this relationship' code.
+        return False
+
 
 class SlugRelatedField(RelatedField):
     """
@@ -349,7 +345,8 @@ def to_internal_value(self, data):
         ]
 
     def get_attribute(self, instance):
-        return self.child_relation.get_iterable(instance, self.source_attrs)
+        relationship = get_attribute(instance, self.source_attrs)
+        return relationship.all() if (hasattr(relationship, 'all')) else relationship
 
     def to_representation(self, iterable):
         return [

tests/test_relations_hyperlink.py

@@ -89,7 +89,14 @@ def test_many_to_many_retrieve(self):
             {'url': 'http://testserver/manytomanysource/2/', 'name': 'source-2', 'targets': ['http://testserver/manytomanytarget/1/', 'http://testserver/manytomanytarget/2/']},
             {'url': 'http://testserver/manytomanysource/3/', 'name': 'source-3', 'targets': ['http://testserver/manytomanytarget/1/', 'http://testserver/manytomanytarget/2/', 'http://testserver/manytomanytarget/3/']}
         ]
-        self.assertEqual(serializer.data, expected)
+        with self.assertNumQueries(4):
+            self.assertEqual(serializer.data, expected)
+
+    def test_many_to_many_retrieve_prefetch_related(self):
+        queryset = ManyToManySource.objects.all().prefetch_related('targets')
+        serializer = ManyToManySourceSerializer(queryset, many=True, context={'request': request})
+        with self.assertNumQueries(2):
+            serializer.data
 
     def test_reverse_many_to_many_retrieve(self):
         queryset = ManyToManyTarget.objects.all()
@@ -99,7 +106,8 @@ def test_reverse_many_to_many_retrieve(self):
             {'url': 'http://testserver/manytomanytarget/2/', 'name': 'target-2', 'sources': ['http://testserver/manytomanysource/2/', 'http://testserver/manytomanysource/3/']},
             {'url': 'http://testserver/manytomanytarget/3/', 'name': 'target-3', 'sources': ['http://testserver/manytomanysource/3/']}
         ]
-        self.assertEqual(serializer.data, expected)
+        with self.assertNumQueries(4):
+            self.assertEqual(serializer.data, expected)
 
     def test_many_to_many_update(self):
         data = {'url': 'http://testserver/manytomanysource/1/', 'name': 'source-1', 'targets': ['http://testserver/manytomanytarget/1/', 'http://testserver/manytomanytarget/2/', 'http://testserver/manytomanytarget/3/']}
@@ -197,7 +205,8 @@ def test_foreign_key_retrieve(self):
             {'url': 'http://testserver/foreignkeysource/2/', 'name': 'source-2', 'target': 'http://testserver/foreignkeytarget/1/'},
             {'url': 'http://testserver/foreignkeysource/3/', 'name': 'source-3', 'target': 'http://testserver/foreignkeytarget/1/'}
         ]
-        self.assertEqual(serializer.data, expected)
+        with self.assertNumQueries(1):
+            self.assertEqual(serializer.data, expected)
 
     def test_reverse_foreign_key_retrieve(self):
         queryset = ForeignKeyTarget.objects.all()
@@ -206,7 +215,8 @@ def test_reverse_foreign_key_retrieve(self):
             {'url': 'http://testserver/foreignkeytarget/1/', 'name': 'target-1', 'sources': ['http://testserver/foreignkeysource/1/', 'http://testserver/foreignkeysource/2/', 'http://testserver/foreignkeysource/3/']},
             {'url': 'http://testserver/foreignkeytarget/2/', 'name': 'target-2', 'sources': []},
         ]
-        self.assertEqual(serializer.data, expected)
+        with self.assertNumQueries(3):
+            self.assertEqual(serializer.data, expected)
 
     def test_foreign_key_update(self):
         data = {'url': 'http://testserver/foreignkeysource/1/', 'name': 'source-1', 'target': 'http://testserver/foreignkeytarget/2/'}

tests/test_relations_pk.py

@@ -71,6 +71,12 @@ def test_many_to_many_retrieve(self):
         with self.assertNumQueries(4):
             self.assertEqual(serializer.data, expected)
 
+    def test_many_to_many_retrieve_prefetch_related(self):
+        queryset = ManyToManySource.objects.all().prefetch_related('targets')
+        serializer = ManyToManySourceSerializer(queryset, many=True)
+        with self.assertNumQueries(2):
+            serializer.data
+
     def test_reverse_many_to_many_retrieve(self):
         queryset = ManyToManyTarget.objects.all()
         serializer = ManyToManyTargetSerializer(queryset, many=True)
@@ -188,6 +194,12 @@ def test_reverse_foreign_key_retrieve(self):
         with self.assertNumQueries(3):
             self.assertEqual(serializer.data, expected)
 
+    def test_reverse_foreign_key_retrieve_prefetch_related(self):
+        queryset = ForeignKeyTarget.objects.all().prefetch_related('sources')
+        serializer = ForeignKeyTargetSerializer(queryset, many=True)
+        with self.assertNumQueries(2):
+            serializer.data
+
     def test_foreign_key_update(self):
         data = {'id': 1, 'name': 'source-1', 'target': 2}
         instance = ForeignKeySource.objects.get(pk=1)

tests/test_relations_slug.py

@@ -54,7 +54,14 @@ def test_foreign_key_retrieve(self):
             {'id': 2, 'name': 'source-2', 'target': 'target-1'},
             {'id': 3, 'name': 'source-3', 'target': 'target-1'}
         ]
-        self.assertEqual(serializer.data, expected)
+        with self.assertNumQueries(4):
+            self.assertEqual(serializer.data, expected)
+
+    def test_foreign_key_retrieve_select_related(self):
+        queryset = ForeignKeySource.objects.all().select_related('target')
+        serializer = ForeignKeySourceSerializer(queryset, many=True)
+        with self.assertNumQueries(1):
+            serializer.data
 
     def test_reverse_foreign_key_retrieve(self):
         queryset = ForeignKeyTarget.objects.all()
@@ -65,6 +72,12 @@ def test_reverse_foreign_key_retrieve(self):
         ]
         self.assertEqual(serializer.data, expected)
 
+    def test_reverse_foreign_key_retrieve_prefetch_related(self):
+        queryset = ForeignKeyTarget.objects.all().prefetch_related('sources')
+        serializer = ForeignKeyTargetSerializer(queryset, many=True)
+        with self.assertNumQueries(2):
+            serializer.data
+
     def test_foreign_key_update(self):
         data = {'id': 1, 'name': 'source-1', 'target': 'target-2'}
         instance = ForeignKeySource.objects.get(pk=1)