The “Visualize Models” plugin By Nils Franzen will generate .png images from the Rails model files :
(click to enlarge) :
This plugin depends on GraphViz, which you can find here.
From your rails application root, run :
ruby script/plugin install
svn://rubyforge.org//var/svn/visualizemodels/visualize_models
rake visualize_models
You’ll then find your .png images in the “doc” directory.