A tool to automatically generate a UML diagram for a Rails Application

Looking for a good tool/gem that can automatically generate a nice looking UML diagram for an existing rails application.

(Im imagining such a tool would read the schema.rb file and then scan the models for relationships)

Which tool is most useful to create a UML use case diagram?

You can create a UML use case diagram in Visio to summarize how users (or actors) interact with a system, such as a software application.

How do you create a class diagram automatically?

To build a class diagram, all you have to do is launch the Smartdraw's Automatic Class Diagram extension and point it at your repository. Load your source code and choose which files you want to include in your diagram. Click "Build Diagram" and you're done.

For Rails 3, try Railroady, it's maintained as well.