Added a test case for PK relations and a possible fix

rest_framework/relations.py

@@ -13,7 +13,9 @@
 from django.utils.translation import ugettext_lazy as _
 
 from rest_framework.compat import OrderedDict
-from rest_framework.fields import Field, empty, get_attribute
+from rest_framework.fields import (
+    Field, empty, get_attribute, is_simple_callable
+)
 from rest_framework.reverse import reverse
 from rest_framework.utils import html
 
@@ -106,7 +108,12 @@ def get_attribute(self, instance):
             # 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]))
+
+                value = instance.serializable_value(self.source_attrs[-1])
+                if is_simple_callable(value):
+                    value = value().pk
+
+                return PKOnlyObject(pk=value)
             except AttributeError:
                 pass
 

tests/test_relations_pk.py

@@ -30,6 +30,16 @@ class Meta:
         fields = ('id', 'name', 'sources')
 
 
+class ForeignKeyTargetCallableSourceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ForeignKeyTarget
+        fields = ('id', 'name', 'first_source')
+
+    first_source = serializers.PrimaryKeyRelatedField(
+        read_only=True,
+        source='get_first_source')
+
+
 class ForeignKeySourceSerializer(serializers.ModelSerializer):
     class Meta:
         model = ForeignKeySource
@@ -450,3 +460,21 @@ def test_reverse_foreign_key_retrieve_with_null(self):
             {'id': 2, 'name': 'target-2', 'nullable_source': 1},
         ]
         self.assertEqual(serializer.data, expected)
+
+
+class PKRelationCallableSourceTests(TestCase):
+
+    def setUp(self):
+        self.target = ForeignKeyTarget.objects.create(name='target-1')
+        self.first_source = ForeignKeySource.objects.create(id=10, name='source-1', target=self.target)
+        ForeignKeySource.objects.create(name='source-2', target=self.target)
+
+    def test_relation_field_callable_source(self):
+        serializer = ForeignKeyTargetCallableSourceSerializer(self.target)
+        expected = {
+            'id': 1,
+            'name': 'target-1',
+            'first_source': 10,
+        }
+        with self.assertNumQueries(1):
+            self.assertEqual(serializer.data, expected)