如何验证模型中的方法参数？

Question

这是我的尝试：

class Answer < ActiveRecord::Base
validate :upd
def upd
 unless question_id.nil? || status.nil?
    redirect_to :action => 'index'
 else return true
end

def update_status(question_id, status)
 if question.valid? && status.valid?
   @update = Question.find(question_id)
   @update.update_attributes(:question_status => status)
 end
end

end

我得到未定义的局部变量或方法`status'