Fix broken depth argument

rest_framework/serializers.py

@@ -388,7 +388,10 @@ class ModelSerializer(Serializer):
         """
         Creates a default instance of a nested relational field.
         """
-        return ModelSerializer()
+        class NestedModelSerializer(ModelSerializer):
+            class Meta:
+                model = model_field.rel.to
+        return NestedModelSerializer()
 
     def get_related_field(self, model_field, to_many=False):
         """

rest_framework/tests/pk_relations.py

@@ -136,6 +136,7 @@ class PrimaryKeyManyToManyTests(TestCase):
         ]
         self.assertEquals(serializer.data, expected)
 
+
 class PrimaryKeyForeignKeyTests(TestCase):
     def setUp(self):
         target = ForeignKeyTarget(name='target-1')