RailRoad is a class diagrams generator for Ruby on Rails applications.1
$ sudo apt-get install graphviz
$ sudo gem install railroad
Generating models diagram in .dot file format and converting it to png:
$ railroad -o models.dot -M
$ dot -Tpng models.dot > models.png
Models diagram in svg format.
$ railroad -M | dot -Tsvg > models.svg
Models diagram with all classes showing inheritance relations
$ railroad -a -i -o full_models.dot -M
Controllers diagram in png format.
$ railroad -C | neato -Tpng > controllers.png
1 Railroad gem
Graphviz
Ruby on rails diagram generator – RailRoad