The original MongoDB implementation of the modulestore.
_id
pointers to its children.common/lib/xmodule/xmodule/modulestore/mongo
To render the courseware page, the Old Mongo modulestore loads the root node and two block levels deep. It grabs all of the block's display names and uses those to render the table of contents.
When displaying a section, it will grab an entire subtree of the course corresponding to that section. Each block retrieval is a single MongoDB request.