Reduce the number of queries when walking parents in MongoModuleStore by…

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)
         ],
     }