example

state_machine> t = TrafficLight.new
state_machine> t.change_color; t.state_name
=> :red_yellow
state_machine> t.change_color; t.state_name
=> :green
state_machine> t.change_color; t.state_name
=> :yellow
state_machine> t.change_color; t.state_name
=> :red
state_machine> t.change_color; t.state_name
=> :red_yellow
state_machine> t.can_set_red?
=> false
state_machine> t.can_set_green?
=> true
state_machine> t.can_set_yellow?
=> false
state_machine> t.set_green
=> true
state_machine> t.can_set_green?
=> false
state_machine> t.can_set_red
t.can_set_red_yellow?  t.can_set_red?         
state_machine> t.can_set_red?
=> false
state_machine> t.can_set_yellow?
=> true
state_machine> 

traffic_light.rb

class TrafficLight < ActiveRecord::Base


  state_machine :state, :initial => :red do

      event :change_color do
        transition :red => :red_yellow
        transition :red_yellow => :green
        transition :green => :yellow
        transition :yellow => :red
      end

      event :set_green do
        transition :red_yellow => :green
      end         
                  
     event :set_yellow do
       transition :green => :yellow
     end

     event :set_red do
       transition :yellow => :red
     end

     event :set_red_yellow do
       transition :red => :red_yellow
     end                 
                         
  end
end