this is strange - the varible name, @default_db, implies that it's a DB instance, but we're passing it to DB.new so it has to be a string.

I looked at #initialize and default_db is undocumented as an option so it's not clear if it's a name or DB instance.
I would suggest:
- determining whether default_db is a String or DB instance
- adding documentation indicating what the #initialize method expects
- Making sure that, if default_db is a DB instance, we pass @default_db.name to DB#initialize here