Let rooms have subrooms called "topics"

Things I'm not sure about: 
-- DJCP told me to look into adjacency lists, 
-- possibly using the [ancestry gem,]( https://github.com/stefankroes/ancestry) which doesn't have specific instructions for rails 4 and scares the crap out of me
-- and to possibly collapse my model hierarchy so that rooms can own other rooms, ( issues with that: rooms could then have null passwords & rooms at the top of the hierarchy wouldn't belong_to any other room )

why the bang?