module ActiveRecord
  module Mixins
    # Including this mixins will record when objects of the class are created in a datetime column called "created_at"
    # and when its updated in another datetime column called "updated_at".
    #
    #   class Bill < ActiveRecord::Base
    #     include ActiveRecord::Mixins::Touch
    #   end
    #
    #   bill = Bill.create("amount" => 100)
    #   bill.created_at # => Time.now at the moment of Bill.create
    #   bill.updated_at # => Time.now at the moment of Bill.create
    #
    #   bill.update_attribute("amount", 150)
    #   bill.created_at # => Time.now at the moment of Bill.create
    #   bill.updated_at # => Time.now at the moment of bill.update_attribute
    module Touch
      def self.append_features(base)
        super

        base.before_create :touch_on_create
        base.before_update :touch_on_update
      end  

      def touch_on_create
        self.updated_at = (self.created_at ||= Time.now)
      end

      def touch_on_update
        self.updated_at = Time.now
      end
    end
  end
end