title: How call custom class method, when Spree order finalize! method executed
body: <p>I am developing spree extension. What I want to do, is to create new database record when order finalized. I need some suggestion how to do it. 
  How I understand one way is to create custom method in order class, and register new hook (where I should register new hook? in initializer?). 
  Other way is to use Activators, but I don't have any idea how to subscribe events. And where I should put code which subscribe order events. </p>
  
  <pre><code>module Spree
    class Credit &lt; ActiveRecord::Base
       def create_new_line(order) 
          #I need call this method when order finalized    
       end  
    end
  end 
  </code></pre>
  
  <p>I found solution. My order decorator looks like this. </p>
  
  <pre><code>Spree::Order.class_eval do
    register_update_hook :add_user_credits
    def add_user_credits
      if (!self.user.nil? and !self.completed_at.nil?)
        # do some stuff, only for registered users and when order complete
      end
    end
  end
  </code></pre>