Added test, CHANGELOG

Author: Anton-Shutik

CHANGELOG.md

@@ -20,6 +20,7 @@ any parts of the framework not mentioned in the documentation should generally b
 * Don't swallow `filter[]` params when there are several
 * Fix DeprecationWarning regarding collections.abc import in Python 3.7
 * Allow OPTIONS request to be used on RelationshipView.
+* Use DRF code when extracting relation instance. [PR](https://github.com/django-json-api/django-rest-framework-json-api/pull/632)
 
 ## [2.7.0] - 2019-01-14
 

example/tests/unit/test_renderer_class_methods.py

@@ -9,12 +9,13 @@
 
 class ResourceSerializer(serializers.ModelSerializer):
     version = serializers.SerializerMethodField()
+    skipped_field = serializers.ReadOnlyField(source='foobar')
 
     def get_version(self, obj):
         return '1.0.0'
 
     class Meta:
-        fields = ('username',)
+        fields = ('username', 'skipped_field')
         meta_fields = ('version',)
         model = get_user_model()
 
@@ -148,3 +149,20 @@ def test_extract_root_meta_invalid_meta():
     serializer = InvalidExtractRootMetaResourceSerializer()
     with pytest.raises(AssertionError):
         JSONRenderer.extract_root_meta(serializer, {})
+
+
+def test_extract_relation_instance():
+    User = get_user_model()
+    user = User.objects.create(username='foo')
+    serializer = ResourceSerializer(instance=user)
+
+    got = JSONRenderer.extract_relation_instance(
+        field=serializer.fields['username'], resource_instance=user
+    )
+    expected = 'foo'
+    assert got == expected
+
+    got = JSONRenderer.extract_relation_instance(
+        field=serializer.fields['skipped_field'], resource_instance=user
+    )
+    assert got is None

rest_framework_json_api/renderers.py

@@ -299,7 +299,7 @@ def extract_relationships(cls, fields, resource, resource_instance):
         return utils._format_object(data)
 
     @classmethod
-    def extract_relation_instance(cls, field_name, field, resource_instance, serializer):
+    def extract_relation_instance(cls, field, resource_instance):
         """
         Determines what instance represents given relation and extracts it.
 
@@ -351,7 +351,7 @@ def extract_included(cls, fields, resource, resource_instance, included_resource
                     continue
 
             relation_instance = cls.extract_relation_instance(
-                field_name, field, resource_instance, current_serializer
+                field, resource_instance
             )
             if isinstance(relation_instance, Manager):
                 relation_instance = relation_instance.all()