attr_accessor_with_history_answer.rb

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name
    attr_reader attr_name+"_history"
    class_eval %Q{
      def #{attr_name}=(new_value)
        @#{attr_name}_history ||= []
        @#{attr_name}_history << @#{attr_name}
        @#{attr_name} = new_value
      end
    }
  end
end