Reduce the number of queries when walking parents in MongoModuleStore by avoiding cache misses on the data due to missing runs

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

@@ -228,6 +228,14 @@ class CachingDescriptorSystem(MakoDescriptorSystem, EditInfoRuntimeMixin):
         Return an XModule instance for the specified location
         """
         assert isinstance(location, UsageKey)
+
+        if location.run is None:
+            # self.module_data is keyed on locations that have full run information.
+            # If the supplied location is missing a run, then we will miss the cache and
+            # incur an additional query.
+            # TODO: make module_data a proper class that can handle this itself.
+            location = location.replace(course_key=self.modulestore.fill_in_run(location.course_key))
+
         json_data = self.module_data.get(location)
         if json_data is None:
             module = self.modulestore.get_item(location, using_descriptor_system=self)
@@ -258,7 +266,7 @@ class CachingDescriptorSystem(MakoDescriptorSystem, EditInfoRuntimeMixin):
                         else ModuleStoreEnum.Branch.draft_preferred
                     )
                     if parent_url:
-                        parent = BlockUsageLocator.from_string(parent_url)
+                        parent = self._convert_reference_to_key(parent_url)
                 if not parent and category != 'course':
                     # try looking it up just-in-time (but not if we're working with a root node (course).
                     parent = self.modulestore.get_parent_location(

lms/djangoapps/ccx/tests/test_field_override_performance.py

@@ -170,7 +170,7 @@ class TestFieldOverrideMongoPerformance(FieldOverridePerformanceTestCase):
             (26, 7, 19), (132, 7, 131), (592, 7, 537)
         ],
         'ccx': [
-            (24, 35, 47), (132, 331, 455), (592, 1507, 2037)
+            (24, 7, 47), (132, 7, 455), (592, 7, 2037)
         ],
     }