association_grapher.rb

require 'graphviz'

def each_model
  ObjectSpace.each_object(Class) do |klass|
    yield klass if klass.ancestors.include?(ActiveRecord::Base) && klass != ActiveRecord::Base
  end
end

graph_viz = GraphViz::new('Gemfile', {:concentrate => true, :normalize => true, :nodesep => 0.55})
graph_viz.edge[:fontname] = graph_viz.node[:fontname] = 'Arial, Helvetica, SansSerif'
graph_viz.edge[:fontsize] = 12

models = {}
each_model do |model|
  name = model.to_s
  models[name] = graph_viz.add_node(name, { :shape => 'box3d', :fontsize => 16, :style => 'filled', :fillcolor => '#B9B9D5' } )
end

each_model do |model|
  model_name = model.to_s
  model.reflect_on_all_associations.each do |assoc|
    assoc_name = assoc.name.to_s.singularize.camelize
    graph_viz.add_edge(models[model_name], models[assoc_name], { :weight => 2 }) unless models[assoc_name].nil?
  end
end

graph_viz.output( :png => 'activerecord_associations.png' )

From http://patshaughnessy.net/2011/9/17/bundlers-best-kept-secret