Fix for resolving source when accessing sub-attr eg b.c

Author: cosmosgenius

rest_framework_json_api/utils.py

@@ -4,6 +4,7 @@
 import copy
 import inspect
 import warnings
+import operator
 from collections import OrderedDict
 
 import django
@@ -339,7 +340,7 @@ def get_included_serializers(serializer):
 
 def get_relation_instance(resource_instance, source, serializer):
     try:
-        relation_instance = getattr(resource_instance, source)
+        relation_instance = operator.attrgetter(source)(resource_instance)
     except AttributeError:
         # if the field is not defined on the model then we check the serializer
         # and if no value is there we skip over the field completely