返回未定義的方法'each'for nil：NilClass？

Question

這是我的視圖中的代碼：show.html.erb

<ul> 
<% @bullets.each do |r| %> 
    <li><%= r.content %></li> 
<% end %> 
</ul> 

這是我的代碼中，所述控制器：users_controller.rb

if cookies[:bullets].nil? 
    @bullets = Bullet.all.shuffle.first(4) 
    cookies[:bullets] = @bullets.collect(&:id) 
else 
@bullets = [] 
cookies[:bullets].each do |id| 
    @bullets << Bullet.find(id) 
end 
end 

這將返回未定義的方法「各自」的零： NilClass上

<% @bullets.each do |r| %> 

我想知道爲什麼它這樣做，我怎麼能解決這個問題，從命名數據庫（sqlite3的）表發佈4項隨機固定子彈內容「子彈」（列是內容）。

編輯：這是整個控制器：

class StudentsController < ApplicationController 
    #GET/
    def index 
     @students = Student.all 

     respond_to do |format| 
     format.html # index.html.erb 
     format.json { render json: @students } 
     end 
    end 

    #GET /new 
    def new 
     @student = Student.new 
    end 

    #POST 
    def create 
     @student = Student.new(params[:student]) 
      if @student.save 
     render :file => 'app/views/success' 
     else 
     render :file => 'app/views/students/fail' 
     end 
     end 

    #GET /students/{:id} 
    def show 
     @student = Student.find_by_url(params[:id]) 

      respond_to do |format| 
     format.html # show.html.erb 
     format.json { render json: @student } 
      end 
    end 

    #BULLETS Randomizing /students/new.html.erb 
    if cookies[:bullets].nil? 
     @bullets = Bullet.all.shuffle.first(4) 
     cookies[:bullets] = @bullets.collect(&:id) 
    else 
    @bullets = [] 
    cookies[:bullets].each do |id| 
     @bullets << Bullet.find(id) 
    end 
    end 

    #GET /students/1/edit 
    def edit 
     @student = Student.find_by_url(params[:id]) 
    end 

    def update 
     @student = Student.find_by_url(params[:id]) 
     respond_to do |format| 
     if @student.update_attributes(params[:student]) 
      format.html { redirect_to @student, notice: 'Student was successfully updated.'} 
     else 
      format.html { render action: "edit" } 
      format.json { render json: @student.errors, status: :unprocessable_entity } 
     end 
     end 
    end 

    #DELETE 
    def destroy 
    @student = Student.find_by_url(params[:id]) 
    @student.destroy 

     respond_to do |format| 
     format.html { redirect_to students_url } 
     format.json { head :no_content } 
     end 
    end 
end 

編輯＃2：像這樣嗎？

#GET /students/{:id} 
    def show 
     @student = Student.find_by_url(params[:id]) 

     #BULLETS Randomizing /students/show.html.erb 
    if cookies[:bullets].nil? 
     @bullets = Bullet.all.shuffle.first(4) 
     cookies[:bullets] = @bullets.collect(&:id) 
    else 
    @bullets = [] 
    cookies[:bullets].each do |id| 
     @bullets << Bullet.find(id) 
    end 
    end 

      respond_to do |format| 
     format.html # show.html.erb 
     format.json { render json: @student } 
      end 
    end 

Answer 1

看起來應該是：

#GET /students/{:id} 
def show 

    @student = Student.find_by_url(params[:id]) 

    #BULLETS Randomizing /students/new.html.erb 
    if cookies[:bullets].nil? 
    @bullets = Bullet.all.shuffle.first(4) 
    cookies[:bullets] = @bullets.collect(&:id) 
    else 
    # simpler to use an 'in list' for only 4 id's 
    Bullet.where("id in (?)", cookies[:bullets]) 
    end 

    respond_to do |format| 
    format.html # show.html.erb 
    format.json { render json: @student } 
    end 
end 

通知我轉換你的循環在cookies陣列分爲單個語句的「列表」 - 應該簡化生成的SQL查找。

雖然，這是值得商榷的，這個碼應該被推到模型：

class Bullet < ActiveRecord::Base 

    NUM_USER_BULLETS = 4 
    # fetch a random set of 
    def self.user_bullets 
    Bullet.all.shuffle.first(NUM_USER_BULLETS) 
    end 
end 

或者類似的東西。那麼你的控制器就更簡單了：

#GET /students/{:id} 
def show 

    @student = Student.find_by_url(params[:id]) 

    #BULLETS Randomizing /students/new.html.erb 
    if cookies[:bullets].nil? 
    @bullets = Bullet.user_bullets 
    cookies[:bullets] = @bullets.collect(&:id) 
    else 
    # simpler to use an 'in list' for only 4 id's 
    Bullet.where("id in (?)", cookies[:bullets]) 
    end 

    respond_to do |format| 
    format.html # show.html.erb 
    format.json { render json: @student } 
    end 
end 

隨着代碼遷移到你的模型中，你的控制器就更簡單了。

Answer 2

class StudentsController < ApplicationController 
    before_filter :get_bullets, :only => [:show] 


    def show 

    end 

    ... 


    protected 

    def get_bullets 
    #BULLETS Randomizing /students/new.html.erb 
    if cookies[:bullets].nil? 
     @bullets = Bullet.all.shuffle.first(4) 
     cookies[:bullets] = @bullets.collect(&:id) 
    else 
    @bullets = [] 
    cookies[:bullets].each do |id| 
     @bullets << Bullet.find(id) 
    end 
    end 

    end 



end 

Answer 3

我建議重構你的「隨機子彈」的代碼放到一個before_filter爲show行動，並在數據庫中進行隨機化，這將是比加載整個表，並執行在Ruby中隨機化速度快：

class UsersController < ApplicationController 
    before_filter :assign_bullets, :only => :show 

    # ... 

private 

    def assign_bullets 
    if cookies[:bullets] 
     @bullets = cookies[:bullets].map { |id| Bullet.find(id) } 
    else 
     @bullets = Bullet.order('RANDOM()').limit(4) 
     cookies[:bullets] = @bullets.map(&:id) 
    end 
    end 
end