RailRoady is an incredibly awesome gem that spits out UML Diagrams for a Rails application. It currently has support for ActiveRecord, Mongoid, and Datamapper. RailRoady relies on graphviz, you can install it via
$ sudo apt-get install graphviz
Include it in your Gemfile
And then run the following…
$ rake diagram:all
Check the doc folder of your application to find svg’s of both your models and controllers. That’s it!
I found this incredibly helpful when modeling a complex relational database. I’m fairly new to relational databases, so a visualization really helps tie everything together in my brain. I can definitely see myself using this tool when jumping into a large open source Rails project that I’m unfamiliar with.