ruby-on-rails-3 - before_create 回调生成随机数组

Question

当我创建一个随机生成的考试时，我想将所有正确答案存储在一个数组中。我这样做的原因是，当我对考试进行评分时，我想通过将 user_answer 与 correct_answer 数组中的相同元素进行匹配来查看答案是否正确。不幸的是，当我使用回调时，它会将正确答案按随机顺序排列，而我无法正确匹配它们。

      ##controller##
class ExamsController < ApplicationController
  def create
    exam = current_user.exams.create!(test_bank_questions: TestBankQuestion.all.sample(5))
    exam.answers
    redirect_to exam_question_path(exam, '1')
  end
end
      #####Model######
class Exam
  include Mongoid::Document

  before_create :answers  

  field :user_answer, type: Array, default: []
  field :correct_answers_ids, type: Array, default: []

  belongs_to :user
  has_and_belongs_to_many :test_bank_questions


   #### This is where my problem is ####
     #I am trying to get all the id's of the correct answer 
     #and put them in an array when the object is  created

   def answers     
    exam_questions = self.test_bank_questions
      exam_questions.each do |question|
        answer_choices = question.answer_choices
          answer_choices.each do |choice|
            if choice.correct_choice == true
              self.correct_answers_ids << choice.id.to_s
            end
          end
      end

    return correct_answers_ids
  end  

end

    ####Model ####
 class TestBankQuestion
   include Mongoid::Document
   field :question_url, type: String
   embeds_many :answer_choices
   has_and_belongs_to_many :exams
 end

  ###Model ###
 class AnswerChoice
   include Mongoid::Document

   field :choice_url, type: String, default: []
   field :correct_choice, type: Boolean, default: []
   embedded_in :test_bank_question
 end