Question

我现在遇到这个问题太久了，我无法找出问题所在，所以：

    Showing /home/alex/Desktop/personal/app/views/entries/list.html.erb where line #17 raised:

    undefined method `name' for nil:NilClass

Extracted source (around line #17):

14:   <% @entries.each do |entry| %>
15:   <tr>
16:   <td><%= link_to entry.title, :action => "show", :id => entry.id %></td>
17:   <td><%= link_to entry.category.name, :action => "list", :category_id => entry.category.id %></td>
18:   </tr>
19:   <% end %> 
20:

我的观看/条目/ list.html.erb看起来像这样：

<html> 
  <head>
    <title>All Entries</title>
  </head>
  <body>

  <h1>Online Personal Collection- All Entries</h1>
  <table border="1">
  <tr>
  <td width="80%"><p align="center"><i><b>Entry</b></i></td> 
   <td width="20%"><p align="center"><i><b>Category</b></i></td>
  </tr>

  <% @entries.each do |entry| %>
  <tr>
  <td><%= link_to entry.title, :action => "show", :id => entry.id %></td>
  <td><%= link_to entry.category.name, :action => "list", :category_id => entry.category.id %></td>
  </tr>
  <% end %> 


  </table>
  <p><%= link_to "Create new entry", :action => "new" %></p>
   <br />
   <%=link_to "Back to Index", entries_path%>
   <br />
   <%=link_to "Back to Account Info", my_account_path%>
   <br />
   <h3>Enter keyword</h3>
<form action ="search" method="post">
<input name = "key" type="input" />
<input value="Send" type="submit"/>
</form>
</body> 
</html>

模型是这样的：

class Entry < ActiveRecord::Base
  attr_accessible  :comments, :date, :description, :title, :category_id, :category_name
  belongs_to :category
after_create do |entry|
      logger.info "entry created: #{entry.title} #{entry.description}"
end
end




class Category < ActiveRecord::Base
  attr_accessible :name
  has_many :entries
end

和entries_controller：

class EntriesController < ApplicationController
  # GET /entries
  # GET /entries.json
  def index

    @entries = Entry.all

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

  def list
      if params[ :category_id].nil?
                  @entries = Entry.find(:all)
          else
              @entries = Entry.find(:all ,
                                :conditions => ["category_id = ?" , params[ :category_id]])
                   params[ :category_id] = nil

      respond_to do |format|
      format.html # list.html.erb
      format.json { render json: @entry }
    end
  end
  end


  # GET /entries/1
  # GET /entries/1.json
  def show
    @entry = Entry.find(params[:id])
    @category = Category.find(:all)


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

  # GET /entries/new
  # GET /entries/new.json
  def new
    @entry = Entry.new
    @categories= Category.find(:all)

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @entry }
    end
  end

  # GET /entries/1/edit
  def edit
    @entry = Entry.find(params[:id])
    @categories = Category.find(:all)
  end

  # POST /entries
  # POST /entries.json
  def create
    @entry = Entry.new(params[:entry])
    respond_to do |format|
      if @entry.save
        format.html { redirect_to @entry, notice: 'Entry was successfully created.' }
        format.json { render json: @entry, status: :created, location: @entry }
      else
        format.html { render action: "new" }
        format.json { render json: @entry.errors, status: :unprocessable_entity }
      end
    end
  end

  # PUT /entries/1
  # PUT /entries/1.json

现在，如果有人发现问题，并且可以帮助我理解我做错了什么，我将不胜感激。谢谢！亚历克斯。

Answer 1

您有一个没有类别的条目，因此entry.category为零，因此entry.category.name（在此例中）为nil.name，这没有任何意义。通常很好的做法是避免因为这个问题而在这样的关联上链接方法。

Rails具有内置委托，您可以使用它来阻止这种情况：

class Entry < ActiveRecord::Base
  #...
  delegate :name, to: :category, prefix: :category, allow_nil: true
end

这定义了entry.category_name实例方法。在您的视图中，如果该条目不存在任何类别，则不会出现任何类别。您可以阅读有关委派方法和选项here的更多信息。

<强>更新

所以我忽略了你正试图链接到一个nil对象的事实，当你可能真正想做的事情是当对象为零时根本不显示链接。有link_to_if方法（我从未使用过，但可能对你有用）：

<%= link_to_if(entry.category, entry.category_name, :action => "list", :category_id => entry.category.id %>

您仍然需要使用category_name委托方法，因为当第一个参数的计算结果为false时，将打印该名称（不带链接）。

nil的未定义方法名称：NilClass

1 个答案: