Here is a small addition to the acts_as_state_machine plugin to add an on_transition event.

acts_as_state_machine already allows you to specify :enter and :exit events which get triggered when a model enters or leaves a particular state. However, I ran into a situation where there were multiple ways to enter a particular state and I only wanted to call a method whenever a particular event transition happened.

Code is available on github: http://github.com/sudothinker/acts_as_state_machine/tree/master

Thanks to Scott Barron for writing the original acts_as_state_machine.

Here is an example usecase:

class Order < ActiveRecord::Base
   acts_as_state_machine :initial => :opened

   state :opened
   state :closed, :enter => Proc.new {|o| Mailer.send_notice(o)}
   state :returned

   event :close do
     transitions :to => :closed, :from => :opened
   end

   event :return do
     transitions :to => :returned, :from => :closed
   end
  
   event :return_with_email do
     transitions :to => :returned, :from => :closed, :on_transition => :email_user
   end

   def email_user
      Notifier.deliver_order_returned(self)
   end
 end