ruby-on-rails - 为什么我得到一个未定义的方法 `join' 为 nil :NilClass error?

Question

我在项目的“index.html.erb”文件中有这段代码:

<h1>Listing businesses</h1>

<table>
<tr>
    <th>Index</th>
    <th>Name</th>
    <th>Phone Number</th>
    <th>Suite</th>
    <th>Address</th>
    <th>City</th>
<th>Province</th>
<th>Postal Code</th>
<th>Fax</th>
    <th>Latitude</th>
    <th>Longitude</th>
    <th>Website</th>
    </tr>

<% count = 0 %>
<% @businesses.each do |business| %>
<!--<%=business.inspect %> <br> <br>-->
<%count = count.to_i + 1 %> 
<tr>
    <td><%= count %></td>
    <td><%= business.name %></td>
    <td><%= business.phone %></td>
    <td><%= business.suite %></td>
    <td><%= business.address %></td>
    <td><%= business.city %></td>
    <td><%= business.province %></td>
    <td><%= business.phone %></td>
    <td><%= business.postalcode %></td>
    <td><%= business.fax %></td>
    <td><%= business.latitude %></td>
    <td><%= business.longitude %></td>
    <td><%= business.website %></td>
    <td><%= link_to 'Show', business %></td>
    <td><%= link_to 'Edit', edit_business_path(business) %></td>
    <td><%= link_to 'Destroy', user, confirm: 'Are you sure?', method: :delete %></td>
</tr>
<% end %>
</table>

当我访问这个 url 时:

http://localhost:3000/businesses/

我收到这个错误:

NoMethodError in Businesses#index

Showing /Users/AM/Documents/RailsWS/cmdLineWS/Directory/app/views/businesses/index.html.erb where line #37 raised:

undefined method `join' for nil:NilClass
Extracted source (around line #37):

34:     <td><%= business.latitude %></td>
35:     <td><%= business.longitude %></td>
36:     <td><%= business.website %></td>
37:     <td><%= link_to 'Show', business %></td>
38:     <td><%= link_to 'Edit', edit_business_path(business) %></td>
39:     <td><%= link_to 'Destroy', user, confirm: 'Are you sure?', method: :delete %></td>
40:   </tr>
Rails.root: /Users/AM/Documents/RailsWS/cmdLineWS/Directory

知道发生了什么事吗？

这里是完整的调用栈:

activemodel (3.2.1) lib/active_model/conversion.rb:52:in `to_param'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:417:in `block in <class:Generator>'
journey (1.0.3) lib/journey/formatter.rb:31:in `call'
journey (1.0.3) lib/journey/formatter.rb:31:in `block (2 levels) in generate'
journey (1.0.3) lib/journey/formatter.rb:30:in `each'
journey (1.0.3) lib/journey/formatter.rb:30:in `block in generate'
journey (1.0.3) lib/journey/formatter.rb:58:in `match_route'
journey (1.0.3) lib/journey/formatter.rb:16:in `generate'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:510:in `generate'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:551:in `generate'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:576:in `url_for'
actionpack (3.2.1) lib/action_dispatch/routing/url_for.rb:148:in `url_for'
actionpack (3.2.1) lib/action_view/helpers/url_helper.rb:107:in `url_for'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:202:in `business_path'
app/views/businesses/index.html.erb:37:in `block in _app_views_businesses_index_html_erb___1138300221443858101_2152537040'
app/views/businesses/index.html.erb:20:in `each'
app/views/businesses/index.html.erb:20:in `_app_views_businesses_index_html_erb___1138300221443858101_2152537040'
actionpack (3.2.1) lib/action_view/template.rb:143:in `block in render'
activesupport (3.2.1) lib/active_support/notifications.rb:125:in `instrument'
actionpack (3.2.1) lib/action_view/template.rb:141:in `render'
actionpack (3.2.1) lib/action_view/renderer/template_renderer.rb:41:in `block (2 levels) in render_template'
actionpack (3.2.1) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
activesupport (3.2.1) lib/active_support/notifications.rb:123:in `block in instrument'
activesupport (3.2.1) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
activesupport (3.2.1) lib/active_support/notifications.rb:123:in `instrument'
actionpack (3.2.1) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
actionpack (3.2.1) lib/action_view/renderer/template_renderer.rb:40:in `block in render_template'
actionpack (3.2.1) lib/action_view/renderer/template_renderer.rb:48:in `render_with_layout'
actionpack (3.2.1) lib/action_view/renderer/template_renderer.rb:39:in `render_template'
actionpack (3.2.1) lib/action_view/renderer/template_renderer.rb:12:in `render'
actionpack (3.2.1) lib/action_view/renderer/renderer.rb:36:in `render_template'
actionpack (3.2.1) lib/action_view/renderer/renderer.rb:17:in `render'
actionpack (3.2.1) lib/abstract_controller/rendering.rb:109:in `_render_template'
actionpack (3.2.1) lib/action_controller/metal/streaming.rb:225:in `_render_template'
actionpack (3.2.1) lib/abstract_controller/rendering.rb:103:in `render_to_body'
actionpack (3.2.1) lib/action_controller/metal/renderers.rb:28:in `render_to_body'
actionpack (3.2.1) lib/action_controller/metal/compatibility.rb:50:in `render_to_body'
actionpack (3.2.1) lib/abstract_controller/rendering.rb:88:in `render'
actionpack (3.2.1) lib/action_controller/metal/rendering.rb:16:in `render'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:40:in `block (2 levels) in render'
activesupport (3.2.1) lib/active_support/core_ext/benchmark.rb:5:in `block in ms'
/Users/AM/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/benchmark.rb:310:in `realtime'
activesupport (3.2.1) lib/active_support/core_ext/benchmark.rb:5:in `ms'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:40:in `block in render'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:83:in `cleanup_view_runtime'
activerecord (3.2.1) lib/active_record/railties/controller_runtime.rb:24:in `cleanup_view_runtime'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:39:in `render'
actionpack (3.2.1) lib/action_controller/metal/implicit_render.rb:10:in `default_render'
actionpack (3.2.1) lib/action_controller/metal/mime_responds.rb:268:in `block in retrieve_response_from_mimes'
actionpack (3.2.1) lib/action_controller/metal/mime_responds.rb:195:in `call'
actionpack (3.2.1) lib/action_controller/metal/mime_responds.rb:195:in `respond_to'
app/controllers/businesses_controller.rb:5:in `index'
actionpack (3.2.1) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (3.2.1) lib/abstract_controller/base.rb:167:in `process_action'
actionpack (3.2.1) lib/action_controller/metal/rendering.rb:10:in `process_action'
actionpack (3.2.1) lib/abstract_controller/callbacks.rb:18:in `block in process_action'
activesupport (3.2.1) lib/active_support/callbacks.rb:414:in `_run__4378774651029744650__process_action__1974792849221574797__callbacks'
activesupport (3.2.1) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.1) lib/active_support/callbacks.rb:385:in `_run_process_action_callbacks'
activesupport (3.2.1) lib/active_support/callbacks.rb:81:in `run_callbacks'
actionpack (3.2.1) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (3.2.1) lib/action_controller/metal/rescue.rb:29:in `process_action'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:30:in `block in process_action'
activesupport (3.2.1) lib/active_support/notifications.rb:123:in `block in instrument'
activesupport (3.2.1) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
activesupport (3.2.1) lib/active_support/notifications.rb:123:in `instrument'
actionpack (3.2.1) lib/action_controller/metal/instrumentation.rb:29:in `process_action'
actionpack (3.2.1) lib/action_controller/metal/params_wrapper.rb:205:in `process_action'
activerecord (3.2.1) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
actionpack (3.2.1) lib/abstract_controller/base.rb:121:in `process'
actionpack (3.2.1) lib/abstract_controller/rendering.rb:45:in `process'
actionpack (3.2.1) lib/action_controller/metal.rb:203:in `dispatch'
actionpack (3.2.1) lib/action_controller/metal/rack_delegation.rb:14:in `dispatch'
actionpack (3.2.1) lib/action_controller/metal.rb:246:in `block in action'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:66:in `call'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:66:in `dispatch'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:30:in `call'
journey (1.0.3) lib/journey/router.rb:68:in `block in call'
journey (1.0.3) lib/journey/router.rb:56:in `each'
journey (1.0.3) lib/journey/router.rb:56:in `call'
actionpack (3.2.1) lib/action_dispatch/routing/route_set.rb:589:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
rack (1.4.1) lib/rack/etag.rb:23:in `call'
rack (1.4.1) lib/rack/conditionalget.rb:25:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/head.rb:14:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/params_parser.rb:21:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/flash.rb:242:in `call'
rack (1.4.1) lib/rack/session/abstract/id.rb:205:in `context'
rack (1.4.1) lib/rack/session/abstract/id.rb:200:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/cookies.rb:338:in `call'
activerecord (3.2.1) lib/active_record/query_cache.rb:64:in `call'
activerecord (3.2.1) lib/active_record/connection_adapters/abstract/connection_pool.rb:443:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/callbacks.rb:28:in `block in call'
activesupport (3.2.1) lib/active_support/callbacks.rb:405:in `_run__539745845193235082__call__2953064504705011211__callbacks'
activesupport (3.2.1) lib/active_support/callbacks.rb:405:in `__run_callback'
activesupport (3.2.1) lib/active_support/callbacks.rb:385:in `_run_call_callbacks'
activesupport (3.2.1) lib/active_support/callbacks.rb:81:in `run_callbacks'
actionpack (3.2.1) lib/action_dispatch/middleware/callbacks.rb:27:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/reloader.rb:65:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/remote_ip.rb:31:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/debug_exceptions.rb:16:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/show_exceptions.rb:56:in `call'
railties (3.2.1) lib/rails/rack/logger.rb:26:in `call_app'
railties (3.2.1) lib/rails/rack/logger.rb:16:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/request_id.rb:22:in `call'
rack (1.4.1) lib/rack/methodoverride.rb:21:in `call'
rack (1.4.1) lib/rack/runtime.rb:17:in `call'
activesupport (3.2.1) lib/active_support/cache/strategy/local_cache.rb:72:in `call'
rack (1.4.1) lib/rack/lock.rb:15:in `call'
actionpack (3.2.1) lib/action_dispatch/middleware/static.rb:53:in `call'
railties (3.2.1) lib/rails/engine.rb:479:in `call'
railties (3.2.1) lib/rails/application.rb:220:in `call'
rack (1.4.1) lib/rack/content_length.rb:14:in `call'
railties (3.2.1) lib/rails/rack/log_tailer.rb:14:in `call'
rack (1.4.1) lib/rack/handler/webrick.rb:59:in `service'
/Users/AM/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/webrick/httpserver.rb:111:in `service'
/Users/AM/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/webrick/httpserver.rb:70:in `run'
/Users/AM/.rvm/rubies/ruby-1.9.2-p290/lib/ruby/1.9.1/webrick/server.rb:183:in `block in start_thread'

应用程序的模型是名为 business.rb 的文件，它包含以下内容:

class Business < ActiveRecord::Base
  establish_connection "Listings_development"
end

Answer 1

由于您的表似乎没有主键，请阅读 Rails Migrations Guide并编写一个将 id 列添加到您的表中。

def change
  add_column :business, :id, :primary_key
end

---

如所述in the documentation of the to_param method ，如果您的对象尚未持久化，它会返回 nil。

这是因为指向不在数据库中的对象的链接实际上是断开的链接。在尝试链接到对象之前，请确保已保存您的对象。

---

<td><%= link_to 'Show', business %></td>

这可能是问题所在。 You must link to an URL ，而不是模型。使用助手获取引用对象的助手:

<td><%= link_to 'Show', business_path(business) %></td>

我想这里也会出现同样的问题:

<td><%= link_to 'Destroy', user, ... %></td>

您应该将该行替换为:

<td><%= link_to 'Destroy', destroy_user_path(user), ... %></td>