Save the resource after processing relationships

- Prevents issues saving one-to-many relationships

drf_jsonapi/mixins.py

@@ -189,14 +189,14 @@ def partial_update(self, request, *args, **kwargs):
         if not serializer.is_valid():
             return self.error_response(Error.parse_validation_errors(serializer.errors))
 
-        resource = serializer.save()
-
         # Check for relationships and process them
         if "relationships" in request.data["data"]:
             self.process_relationships(
                 request.data["data"]["relationships"], resource, request
             )
 
+        serializer.save()
+
         self.document.instance.data = serializer.data
 
         return Response(self.document.data)