Visualize Rails schema.rb with tool/plugin/script
Check out RailRoad.
It creates diagrams for models and controllers. I bet it will do what you want. It produces DOT files which can be viewed with Graphviz or other programs.
I've had some success with it, but when you have a ton of models, you may need to do some manual rearrangement to get a decent looking diagram.
(source: rubyforge.org)
Checkout RailRoady for a Rails 3 project. It's forked from RailRoad.