How can I do drop down dependencies?

First: Apologies if already addressed.

i have two situations where i need dropdown/select dependencies and wondering if anyone can help with a simple way of handling this using Rails, gem, mininal js.

1. using Ancestry gem and parent/child setup
2. using one to many association
   

i came across subdivision_select gem and like the simplicity of this.

@chris maybe a screencast on this?

Any help/direction is much appreciated

cheers
Josh