Use DRF code to fetch related instance

Author: Anton-Shutik

rest_framework_json_api/renderers.py

@@ -301,30 +301,10 @@ def extract_relation_instance(cls, field_name, field, resource_instance, seriali
         """
         Determines what instance represents given relation and extracts it.
 
-        Relation instance is determined by given field_name or source configured on
-        field. As fallback is a serializer method called with name of field's source.
+        Relation instance is determined exactly same way as it determined
+        in parent serializer
         """
-        relation_instance = None
-
-        try:
-            relation_instance = getattr(resource_instance, field_name)
-        except AttributeError:
-            try:
-                # For ManyRelatedFields if `related_name` is not set
-                # we need to access `foo_set` from `source`
-                relation_instance = getattr(resource_instance, field.child_relation.source)
-            except AttributeError:
-                if hasattr(serializer, field.source):
-                    serializer_method = getattr(serializer, field.source)
-                    relation_instance = serializer_method(resource_instance)
-                else:
-                    # case when source is a simple remap on resource_instance
-                    try:
-                        relation_instance = getattr(resource_instance, field.source)
-                    except AttributeError:
-                        pass
-
-        return relation_instance
+        return field.get_attribute(resource_instance)
 
     @classmethod
     def extract_included(cls, fields, resource, resource_instance, included_resources,