Today we can't have an associated entity called Collection, because Model.collection returns the mongo collection from the current session.
So, if we try to get the book's collection with Book.collection, it won't work. There is any way to workaround on this? My suggestion would be change the collection method to a name that doesn't represents an abstraction so hight, I mean, change to collection_session or something like that, I really don't know a good name. Do I make myself clear?