additional courseware view optimizations. Do a 'depth' fetch on the selected…

additional courseware view optimizations. Do a 'depth' fetch on the selected section so that it does a more efficient set of queries to the database. Also, in the CachingDescriptorSystem, if we have a 'cache miss', when we do the actual fetch (which creates a new 'system'), keep that fetched data around in our own collection, in case it is queried again

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

@@ -64,7 +64,10 @@ class CachingDescriptorSystem(MakoDescriptorSystem):
         location = Location(location)
         json_data = self.module_data.get(location)
         if json_data is None:
-            return self.modulestore.get_item(location)
+            module = self.modulestore.get_item(location)
+            if module is not None:
+                self.module_data.update(module.system.module_data)
+            return module
         else:
             # load the module and apply the inherited metadata
             try:

lms/djangoapps/courseware/views.py

@@ -306,6 +306,10 @@ def index(request, course_id, chapter=None, section=None,
                 # Specifically asked-for section doesn't exist
                 raise Http404
 
+            # cdodge: this looks silly, but let's refetch the section_descriptor with depth=None
+            # which will prefetch the children more efficiently than doing a recursive load
+            section_descriptor = modulestore().get_instance(course.id, section_descriptor.location, depth=None)
+
             # Load all descendants of the section, because we're going to display its
             # html, which in general will need all of its children
             section_module_cache = StudentModuleCache.cache_for_descriptor_descendents(