it appears we are taking one too many round trips to do when pre-fetching…

it appears we are taking one too many round trips to do when pre-fetching children. This can be very expensive as the tree gets wider the deeper we go. For example, in courseware we want depth=2 (course, chapter, sequential). But looking at log output we were also getting verticals, which there can be a lot of. This should cut down on the total data we are grabbing from the DB.

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

@@ -366,6 +366,9 @@ class MongoModuleStore(ModuleStoreBase):
                 children.extend(item.get('definition', {}).get('children', []))
                 data[Location(item['location'])] = item
 
+            if depth == 0:
+                break;
+
             # Load all children by id. See
             # http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24or
             # for or-query syntax