Pages＃中的NoMethodError创建

Question

嗨，大家好我是ruby on rails的新手，当我尝试添加新页面时，我遇到了错误。当我点击创建时，这就是出现的内容

  

当你没有期待它时，你有一个零对象！你可能有   期待一个Array的实例。评估时发生错误   nil.collect

 class PagesController < ApplicationController

    layout 'admin'#apply the admin.html layout

    before_filter :confirm_logged_in #confirms if the user is logged in before entering the   page
    before_filter :find_subject

    def index
      list
      render('list')#runs pages/list.html
    end

    def list
    @pages = Page.where(:subject_id => @subject.id)#for the list view it finds the products   sorted,all of them
    end

    def show
     @page = Page.find(params[:id])#it finds the information of the product by its id and shows it in the show.html
    end

    def new
    @page = Page.new(:subject_id => @subject.id)#it creates a new page(product) in the corresponding subject(category) 
    @page_count = @subject.pages.size + 1 #the quantity of total (pages)products all of them
    @subjects = Subject.order('position ASC')# shows the subjects ordered ascending
      end

      def create
         #new_position = params[:page].delete(:position)

        @page = Page.new(params[:page]) #instantiate a new object using form parameters
      puts "Im here"
      if @page.save# save the object
      #@page.move_to_position(new_position)

        flash[:notice] = "Product created."
      redirect_to(:action => 'list', :subject_id => @page.subject_id)# if save succeeds, redirect to the list action
    else     
      @page_count = @subject.pages.size + 1 # products quantity is updated
      @subjects = Subject.order('position ASC')
      render('new') # if save fails, redisplay the form so user can fix problems
    end
  end

  def edit
    @page = Page.find(params[:id])# it find the page to be edited by its id
    @page_count = @subject.pages.size 
    @subjects = Subject.order('position ASC')
  end

  def update
    new_position = params[:page].delete(:position)

    @page = Page.find(params[:id])# find object using parameters

    if @page.update_attributes(params[:page])# update the object
      @page.move_to_position(new_position)

      flash[:notice] = "Page updated."
      redirect_to(:action => 'show', :id => @page.id, :subject_id => @page.subject_id)# if update succeeds, redirect to the list action
    else

      @page_count = @subject.pages.size
      @subjects = Subject.order('position ASC')
      render('edit')# if save fails, redisplay the form so user can fix problems
    end
  end

  def delete
    @page = Page.find(params[:id])#it finds the page by the id to be deleted
  end

  def destroy
    page = Page.find(params[:id])#it finds the page by the id to be destroyed
    page.move_to_position(nil)
    page.destroy
    flash[:notice] = "Product destroyed."
    redirect_to(:action => 'list', :subject_id => @subject.id)# redirect the user to the pages/list.html according to the corresponding subject(category)
  end

  private
   def find_subject
    if params[:subject_id]
      @subject = Subject.find_by_id(params[:subject_id])
    end
  end


end

我可以添加没有任何问题的主题，我可以添加管理员用户而不会出现问题与页面有关。

class SubjectsController < ApplicationController

  layout 'admin'#apply the admin.html layout

  before_filter :confirm_logged_in #confirms if the user is logged in before entering the page

  def index
    list
    render('list')#runs subjects/list.html all list.html are different
  end

  def list
    @subjects = Subject.order("subjects.position ASC")#for the list view it finds the categories sorted,all of them
  end

  def show
    @subject = Subject.find(params[:id])
  end

  def new
    @subject = Subject.new # creates a new subject(category)
    @subject_count = Subject.count + 1
  end

  def create
    new_position = params[:subject].delete(:position)

    @subject = Subject.new(params[:subject]) # instantiate a new object using form parameters

    if @subject.save #save the object

      @subject.move_to_position(new_position)

      flash[:notice] = "Category created."
      redirect_to(:action => 'list')# if save succeeds, redirect to list.html
    else

      @subject_count = Subject.count + 1
      render('new')#If save fails, redisplay the form so user can fix problems
    end
    end


  def edit
    @subject = Subject.find(params[:id])# it finds the subject to be edited
    @subject_count = Subject.count
  end

  def update
    new_position = params[:subject].delete(:position)#if updated it delete the current position

    @subject = Subject.find(params[:id])# find object using form parameters

    if @subject.update_attributes(params[:subject])#update the object(subject)
      @subject.move_to_position(new_position)#new position assigned

      flash[:notice] = "Subject updated."
      redirect_to(:action => 'show', :id => @subject.id)# if update succeeds, redirect to the list action
    else

      @subject_count = Subject.count
      render('edit')# if save fails, redisplay the form so user can fix problems
    end
  end

  def delete
    @subject = Subject.find(params[:id])#it finds thesubjectby the id to be deleted
  end

  def destroy
    subject = Subject.find(params[:id])#it finds the Subject(category)by the id to be destroyed
    subject.move_to_position(nil)
    subject.destroy
    flash[:notice] = "Subject destroyed."
    redirect_to(:action => 'list')# redirected to the list.html 
  end

end

这是_form.html.erb，根据框架，错误应该是签名的 提取的来源（第6行）：

3: <table summary="Products form fields">
4:   <tr>
5:     <th><%= f.label(:subject_id, "Category") %></th>
6:  <td><%= f.select(:subject_id, @subjects.collect {|s| [s.name, s.id]}) %></td>  </tr>

<%= error_messages_for(@pages) %>

<table summary="Products form fields">
  <tr>
    <th><%= f.label(:subject_id, "Category") %></th>
    <td><%= f.select(:subject_id, @subjects.collect {|s| [s.name, s.id]}) %></td>  </tr>
  <tr>
    <th><%= f.label(:product_name) %></th>
    <td><%= f.text_field(:product_name) %></td>
  </tr>
  <tr>
    <th><%= f.label(:description) %></th>
    <td><%= f.text_area(:description) %></td>
  </tr>
  <tr>
    <th><%= f.label(:min_description) %></th>
    <td><%= f.text_area(:min_description) %></td>
  </tr>
  <tr>
    <th><%= f.label(:brand) %></th>
    <td><%= f.text_field(:brand) %></td>
    <tr>
    <th><%= f.label(:quantity) %></th>
    <td><%= f.text_field(:quantity) %></td>
  </tr>
  <tr>
    <th><%= f.label(:price) %></th>
    <td><%= f.text_field(:price) %></td>
  </tr>
  <tr>
    <th><%= f.label(:size) %></th>
    <td><%= f.text_field(:size) %></td>
  </tr>
  <tr>
    <th><%= f.label(:head_size) %></th>
    <td><%= f.text_field(:head_size) %></td>
  </tr>
  <tr>
    <th><%= f.label(:racquets_weight) %></th>
    <td><%= f.text_field(:racquets_weight) %></td>
  </tr>
  <tr>
    <th><%= f.label(:category) %></th>
    <td><%= f.text_field(:category) %></td>
  </tr>
  <tr>
    <th><%= f.label(:image_url) %></th>
    <td><%= f.text_field(:image_url) %></td>

  </tr>
</table>

框架跟踪

  

actionpack（3.0.9）lib / action_view / template.rb：135：在send' actionpack (3.0.9) lib/action_view/template.rb:135:in渲染＆＃39;   activesupport（3.0.9）lib / active_support / notifications.rb:54:in   instrument' actionpack (3.0.9) lib/action_view/template.rb:127:in 渲染＆＃39; actionpack（3.0.9）lib / action_view / render / partials.rb：333：in   render_partial' actionpack (3.0.9) lib/action_view/render/partials.rb:262:in渲染＆＃39;的ActiveSupport   （3.0.9）lib / active_support / notifications.rb:52:in instrument' activesupport (3.0.9) lib/active_support/notifications/instrumenter.rb:21:in instrument＆＃39;   activesupport（3.0.9）lib / active_support / notifications.rb:52:in   instrument' actionpack (3.0.9) lib/action_view/render/partials.rb:260:in渲染＆＃39; actionpack（3.0.9）   lib / action_view / render / partials.rb：378：在_render_partial' actionpack (3.0.9) lib/action_view/render/rendering.rb:22:in渲染＆＃39; ActionPack的   （3.0.9）lib / action_view / helpers / capture_helper.rb:40:in capture' actionpack (3.0.9) lib/action_view/helpers/capture_helper.rb:172:in with_output_buffer＆＃39; actionpack（3.0.9）   lib / action_view / helpers / capture_helper.rb：40：capture' actionpack (3.0.9) lib/action_view/helpers/form_helper.rb:545:in fields_for＆＃39;   actionpack（3.0.9）lib / action_view / helpers / form_helper.rb：320：in   form_for' actionpack (3.0.9) lib/action_view/template.rb:135:in 发送＆＃39; actionpack（3.0.9）lib / action_view / template.rb：135：在render' activesupport (3.0.9) lib/active_support/notifications.rb:54:in 工具＆＃39; actionpack（3.0.9）lib / action_view / template.rb：127：in   render' actionpack (3.0.9) lib/action_view/render/rendering.rb:59:in _ render_template＆＃39; activesupport（3.0.9）   lib / active_support / notifications.rb:52:in instrument' activesupport (3.0.9) lib/active_support/notifications/instrumenter.rb:21:in instrument＆＃39; activesupport（3.0.9）   lib / active_support / notifications.rb:52:in instrument' actionpack (3.0.9) lib/action_view/render/rendering.rb:56:in _ render_template＆＃39;   actionpack（3.0.9）lib / action_view / render / rendering.rb:26:in render' actionpack (3.0.9) lib/abstract_controller/rendering.rb:115:in _ render_template＆＃39; actionpack（3.0.9）   lib / abstract_controller / rendering.rb：109：在render_to_body' actionpack (3.0.9) lib/action_controller/metal/renderers.rb:47:in render_to_body＆＃39; actionpack（3.0.9）   lib / action_controller / metal / compatibility.rb：55：在render_to_body' actionpack (3.0.9) lib/abstract_controller/rendering.rb:102:in render_to_string＆＃39; actionpack（3.0.9）   lib / abstract_controller / rendering.rb：93：在render' actionpack (3.0.9) lib/action_controller/metal/rendering.rb:17:in渲染＆＃39; ActionPack的   （3.0.9）lib / action_controller / metal / instrumentation.rb：40：render' activesupport (3.0.9) lib/active_support/core_ext/benchmark.rb:5:in ms＆＃39; C：/RailsInstaller/Ruby1.8.7/lib/ruby/1.8/benchmark.rb：308：在   realtime' activesupport (3.0.9) lib/active_support/core_ext/benchmark.rb:5:in MS＆＃39; actionpack（3.0.9）   lib / action_controller / metal / instrumentation.rb：40：在render' actionpack (3.0.9) lib/action_controller/metal/instrumentation.rb:78:in cleanup_view_runtime＆＃39; activerecord（3.0.9）   LIB / active_record / railties / controller_runtime.rb：15：在   cleanup_view_runtime' actionpack (3.0.9) lib/action_controller/metal/instrumentation.rb:39:in渲染＆＃39; 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  rack-mount（0.6.14）lib / rack / mount / code_generation.rb:93:in   recognize' rack-mount (0.6.14) lib/rack/mount/code_generation.rb:75:in optimized_each＆＃39;机架式   （0.6.14）lib / rack / mount / code_generation.rb:92:in recognize' rack-mount (0.6.14) lib/rack/mount/route_set.rb:139:in call＆＃39;   actionpack（3.0.9）lib / action_dispatch / routing / route_set.rb：493：in   call' actionpack (3.0.9) lib/action_dispatch/middleware/best_standards_support.rb:17:in呼叫＆＃39;   actionpack（3.0.9）lib / action_dispatch / middleware / head.rb:14:in call' rack (1.2.8) lib/rack/methodoverride.rb:24:in call＆＃39; ActionPack的   （3.0.9）lib / action_dispatch / middleware / params_parser.rb：21：call' actionpack (3.0.9) lib/action_dispatch/middleware/flash.rb:182:in 来电＆＃39; actionpack（3.0.9）   lib / action_dispatch / middleware / session / abstract_store.rb：149：call' actionpack (3.0.9) lib/action_dispatch/middleware/cookies.rb:302:in 来电＆＃39; activerecord（3.0.9）lib / active_record / query_cache.rb:32:in   call' activerecord (3.0.9) lib/active_record/connection_adapters/abstract/query_cache.rb:28:in 缓存＆＃39; 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railties（3.0.9）lib / rails / commands.rb:27 script / rails：6：in   `需要＆＃39;脚本/轨道：6

任何形式的帮助。

Answer 1

确保您始终仔细阅读错误，这是了解错误的关键。这里你的错误说：

  

当你没想到它时，你有一个零对象！您可能期望一个Array实例。评估nil.collect

时发生错误

这告诉我们，在代码中的某个位置，您在nil对象上调用collect。由于您没有明确地在任何地方调用nil.collect，因此可以安全地假设您可能正在调用@some_instance_variable_that_is_not_set.collect。从那里你应该看看它抱怨的行号（在这种情况下第6行）并找到你正在调用的任何变量collect，这可能是零。在这里，你会发现@subjects.collect可能是罪魁祸首。将puts @subjects添加到控制器并检查日志以查看输出结果。如果它为零，则表示您未在控制器的create方法中设置该值。

这是您在浏览错误消息时应始终采用的方法。