用于＃的未定义方法`question_answer'

Question

这是我的两个模特

class Answer
  include Mongoid::Document

  belongs_to :question
  has_many :responses
  has_many :validations, :dependent => :delete
end

和

class Question
  include Mongoid::Document

  has_many :answers, :dependent => :delete # it might not always have answers
  belongs_to :correct_answer, :class_name => "Answer", :dependent => :delete
end

这里缺少什么？无法解决它