在开发一个小功能，controller 是 fun_urls_controller.rb，model 是 fun_url.rb Controller 的代码：

#encoding = utf-8
  2   
  3 class FunUrlsController < ApplicationController
  4   def index
  5     @fun_urls = FunUrl.all
  6   end
  7   
  8   def new
  9     @fun_url = FunUrl.new
 10   end 
 11         
 12   def create
 13     @fun_url = FunUrl.new(params[:fun_url])
 14     if @fun_url.save
 15       redirect_to fun_urls_url, :notice => 'URL添加成功'
 16     else
 17       render :action => :new
 18     end
 19   end
 20       
 21   def edit
 22     @fun_url = FunUrl.find(params[:id])
 23   end   
 24         
 25   def update
 26     @fun_url = FunUrl.find(params[:id])
 27     if @fun_url.update_attributes(params[:fun_url])
 28       redirect_to fun_urls_url, :notice => 'URL修改成功'
 29     else
 30       render :action => :edit
 31     end
 32   end
 33 
 34   def destroy
 35     @fun_url = FunUrl.find(params[:id])
 36 
 37     @fun_url.destroy
 38 
 39     redirect_to fun_urls_url, :alert => 'URL删除成功'
 40   end
 41 end

index 页面的代码：

 1 <article class="module width_full">
 2   <header>
 3     <h3 class="tabs_involved">URL列表</h3>
 4     <ul class="tabs">
 5       <li><%= link_to '添加', new_fun_url_path %></li>
 6     </ul>
 7   </header>
 8   <table class="tablesorter" cellspacing="0">
 9     <thead>
10       <tr>
11         <th></th>
12         <th>编码</th>
13         <th>URL</th>
14         <th>方法</th>
15         <th>操作</th>
16       </tr>
17     </thead> 
18     <tbody>
19     <% @fun_urls.each do |fun_url| %>
20       <tr>
21         <td></td>
22         <td><%= fun_url.code %></td>
23         <td><%= fun_url.url %></td>
24         <td><%= fun_url.method %></td>
25         <td>
26           <%= link_to image_tag('icn_edit.png', :border => 0), edit_fun_url_path(fun_url) %>
27         </td>
28       </tr>
29     <% end %>
30     </tbody> 
31   </table>
32 </article>

如果没有<%= link_to image_tag('icn_edit.png', :border => 0), edit_fun_url_path(fun_url) 这句，页面能正常显示，且能列出数据，如果加上这句，就会报异常：

Showing /home/puras/puras/rails/demo/app/views/fun_urls/index.html.erb where line #26 raised:

undefined method `join' for nil:NilClass
Extracted source (around line #26):

23:         <td><%= fun_url.url %></td>
24:         <td><%= fun_url.method %></td>
25:         <td>
26:           <%= link_to image_tag('icn_edit.png', :border => 0), edit_fun_url_path(fun_url) %>
27:         </td>
28:       </tr>
29:     <% end %>

执行 rake routes 后，能查看到查应的 URL 映射：

fun_urls GET    /fun_urls(.:format)            fun_urls#index
                  POST   /fun_urls(.:format)            fun_urls#create
      new_fun_url GET    /fun_urls/new(.:format)        fun_urls#new
     edit_fun_url GET    /fun_urls/:id/edit(.:format)   fun_urls#edit
          fun_url GET    /fun_urls/:id(.:format)        fun_urls#show
                  PUT    /fun_urls/:id(.:format)        fun_urls#update
                  DELETE /fun_urls/:id(.:format)        fun_urls#destroy

后台日志是：

Started GET "/fun_urls" for 127.0.0.1 at 2012-05-18 12:50:30 +0800
Processing by FunUrlsController#index as HTML
  FunUrl Load (0.1ms)  SELECT `fun_urls`.* FROM `fun_urls` 
  Rendered fun_urls/index.html.erb within layouts/application (2.2ms)
Completed 500 Internal Server Error in 5ms

ActionView::Template::Error (undefined method `join' for nil:NilClass):
    23:         <td><%= fun_url.url %></td>
    24:         <td><%= fun_url.method %></td>
    25:         <td>
    26:           <%= link_to 'aa', edit_fun_url_path(fun_url) %>
    27:         </td>
    28:       </tr>
    29:     <% end %>
  app/views/fun_urls/index.html.erb:26:in `block in _app_views_fun_urls_index_html_erb___434002087180833655_70161042647500'
  app/views/fun_urls/index.html.erb:19:in `each'
  app/views/fun_urls/index.html.erb:19:in `_app_views_fun_urls_index_html_erb___434002087180833655_70161042647500'


  Rendered /home/puras/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/_trace.erb (3.6ms)
  Rendered /home/puras/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (1.2ms)
  Rendered /home/puras/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.3/lib/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (10.4ms)

求赐教～～～