optimize the result-set that gets returned from Mongo on metadata inheritence.…

optimize the result-set that gets returned from Mongo on metadata inheritence. We just need the fields which are actually inheritable, so no need to return anything else as it gets filtered out during the computation

common/lib/xmodule/xmodule/modulestore/mongo.py

@@ -267,8 +267,13 @@ class MongoModuleStore(ModuleStoreBase):
                     '_id.course': location.course,
                     '_id.category': {'$in': ['course', 'chapter', 'sequential', 'vertical']}
                 }
-        # we just want the Location, children, and metadata
-        record_filter = {'_id': 1, 'definition.children': 1, 'metadata': 1}
+        # we just want the Location, children, and inheritable metadata
+        record_filter = {'_id': 1, 'definition.children': 1}
+
+        # just get the inheritable metadata since that is all we need for the computation
+        # this minimizes both data pushed over the wire
+        for attr in INHERITABLE_METADATA:
+            record_filter['metadata.{0}'.format(attr)] = 1
 
         # call out to the DB
         resultset = self.collection.find(query, record_filter)