Allows you to set to arbitrary location in database 

(not just specific collection) through collection.sub1.sub2.sub3.etc

Makes it so we can manage a database structure without creating a clutter of different collections
It's confusing that this line is setting the database, yet the method is called get_sub_collection. Is a sub collection a database? I'm un familiar with concept of a sub collection and a quick google search didn't yield much.