ActiveModel::StateMachine

I believe this was pulled from Rails 3 at the last minute because they decided they didn't need a state machine (Strongly disagree, especially considering it was a mixin), not much documentation exists, but it works very nicely.

Add this line to your bundle:

gem 'state_machine', :git => 'git://github.com/ivanvanderbyl/state_machine.git', :require => 'active_model/state_machine'

Traffic light example

class TrafficLight < ActiveRecord::Base
  include ActiveModel::StateMachine

  state_machine do
    state :red
    state :green
    state :yellow

    event :change_color do
      transitions :to => :red,    :from => [:yellow], :on_transition => :catch_runners
      transitions :to => :yellow, :from => [:green]
      transitions :to => :green,  :from => [:red]
    end
  end

  def catch_runners
    puts "That'll be $250."
  end
end

light = TrafficLight.new
light.current_state       #=> :red
light.change_color!       #=> true
light.current_state       #=> :green
light.change_color!       #=> true
light.current_state       #=> :yellow
light.change_color!       #=> true
"That'll be $250."

EnvyLabs also wrote something about it back in the day http://blog.envylabs.com/2009/08/the-rails-state-machine/