method_missing 的问题

想用 method_missing构造动态方法，以"can_" 开头

ruby 代码

class User < ActiveRecord::Base
    def has_role?(role_in_question)
    @_list ||= self.roles.collect(&:name)
    return true if @_list.include?("admin")
    (@_list.include?(role_in_question.to_s) )
  end

    def method_missing(method_id,arg)
    method_name = method_id.id2name
    arr = method_name.split("_")
    if arr[0] == "can"
              has_role?(arg)
    else
        raise NoMethodError,"there is no method:#{method_name}"
    end
  end
end

我跟踪后发现不执行此方法