Exceptions and the observer pattern

card_creator.rb

class CardCreator                                                                        
  include Publisher                                                                      
                                                                                         
  def create(iteration, attributes)                                                      
    card = iteration.cards.build(attributes)                                             
    card.save!                                                                           
  rescue ActiveRecord::RecordInvalid                                                     
    notify_subscribers(:create_failed, card)                                             
  else
    notify_subscribers(:created, card)                                                   
  end                                                                                    
end               

card_persistence_responder.rb

class CardPersistenceResponder < Struct.new(:controller)                                 
  def created(card)                                                                      
    controller.instance_eval do                                                          
      redirect_to planning_path(card.project)                                            
    end                                                                                  
  end                                                                                    
                                                                                         
  def create_failed(card)                                                                
    controller.instance_eval do                                                          
      @title = 'New card'                                                                
      @card = card                                                               
      flash.now[:alert] = 'Your card needs a title'                              
      render :action => 'new'                                                    
    end                                                                          
  end                                                                            
end

cards_controller.rb

class CardsController < ApplicationController
  def create                                                                             
    creator = CardCreator.new                                                            
    creator.add_subscriber(CardPersistenceResponder.new(self))                   
    creator.create(project.backlog, card_params)                                 
  end
end

publisher.rb

module Publisher                                                                         
  def add_subscriber(object)                                                             
    @subscribers ||= []                                                                  
    @subscribers << object                                                               
  end                                                                                    
                                                                                         
  def notify_subscribers(message, *args)                                                 
    return if @subscribers.blank?                                                   
    @subscribers.each do |subscriber|                                               
      subscriber.send(message, *args) if subscriber.respond_to?(message)            
    end
  end                                                                                    
end

Since I made this gist I've written the process up: http://theagileplanner.com/blog/building-agile-planner/refactoring-with-hexagonal-rails