NoMethodError，nil的未定義方法：NilClass

Question

我有一個帶導軌的測驗應用，

當用戶獲得測驗然后檢查quizuser是否仍為空時，如果為空，則創建quizuser，如果不為空將繼續。

模型

class Quizuser < ActiveRecord::Base
 belongs_to :user
 belongs_to :quiz
end

class Quiz < ActiveRecord::Base
 has_many :quizusers, :dependent => :destroy
end

class User < ActiveRecord::Base
 has_many :quizusers, :dependent => :destroy
end

這是錯誤消息

NoMethodError in QuizzesController#start

undefined method `timer' for nil:NilClass

app/controllers/quizzes_controller.rb:29:in `start'

你可以在控制器上看起來像

檢查quizuser后讀取第29行，如果是nil quizuser.create

為什么timer在@check之前執行？

if @ a.first.timer <DateTime.now

   @mu = Mu.find(params[:id])
   @now = DateTime.now
   @end = @now + @mu.quiz.time.minutes
   @check = Quizuser.where(:quiz_id => @mu.quiz.id, :user_id => current_user.id)
   if @check == nil
      @a = Quizuser.create(:quiz_id => @mu.quiz.id, :user_id => current_user.id, :start => @now, :timer => @end) 
      @havequiz = Havequiz.new
      @havequiz.answerusers.build
   else
     @a = Quizuser.where(:quiz_id => @mu.quiz.id, :user_id => current_user.id)
     if @a.first.timer < DateTime.now
       @a.update_column(:grade, 0)
       redirect_to home_subdomain_path, :notice => 'Sorry your quiz time is over.' 
     else
       @havequiz = Havequiz.new
      @havequiz.answerusers.build
     end
   end


<div class="countdown"></div>

<script type="text/javascript">$(document).ready(function() {
    var date = new Date('<%= @a.first.timer %>');
    $('.countdown').countdown({
        until: date,
        format: 'dHMS'
    });
});</script>

Answer 1

問題是您不應檢查查詢結果的可空性。

嘗試更換

if @check == nil

同

if @check.empty?