NoMethodError in（关于belongs_to）

class Category < ActiveRecord::Base
  has_many :books
end
class Book < ActiveRecord::Base
  belongs_to :category
end

edit.rhtml增加了“分类”,可以正常使用。

...
  <p>
  <b>分类</b><br />
  <select name="book[category_id]">
  <% @categories.each do |category|%>
  <option value="<%= category.id %>"
  <%= 'selected' if category.id==@book.category_id %>>
  <%= category.name %>
  </option>
  <% end %>
  </p>
...

index.rhtml（报错：NoMethodError in Books#index 错误的代码：<td><%= book.category.name %></td>
）

<% for book in @books %>
  <tr>
    <td><%=h book.title %></td>
    <td><%=h book.body %></td>
    <td><%= book.category.name %></td>
    <td><%=h book.buydate %></td>
    <td><%= link_to '详细', book %></td>
    <td><%= link_to '编辑', edit_book_path(book) %></td>
    <td><%= link_to '删除', book, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>

book和category已经建立的表关联，照理book应该可以调用category的属性啊，为什么会这样呢？

最好贴上完整的错误信息

NoMethodError in Books#index
Showing books/index.html.erb where line #15 raised:

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.name

Extracted source (around line #15):

12:   <tr>
13:     <td><%=h book.title %></td>
14:     <td><%=h book.body %></td>
15:     <td><%= book.category.name %></td>
16:     <td><%=h book.buydate %></td>
17:     <td><%= link_to '详细', book %></td>
18:     <td><%= link_to '编辑', edit_book_path(book) %></td>



RAILS_ROOT: E:/Work/shu

Application Trace | Framework Trace | Full Trace
app/views/books/index.html.erb:15:in `_run_erb_47app47views47books47index46html46erb'
app/views/books/index.html.erb:11:in `each'
app/views/books/index.html.erb:11:in `_run_erb_47app47views47books47index46html46erb'
app/controllers/books_controller.rb:7:in `index'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:637:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:637:in `compile_and_render_template'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:365:in `render_template'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:316:in `render_file'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1100:in `render_for_file'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:858:in `render_with_no_layout'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:872:in `render_with_no_layout'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/layout.rb:262:in `render_without_benchmark'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
e:/ruby/lib/ruby/1.8/benchmark.rb:293:in `measure'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:131:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:131:in `custom'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:152:in `call'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:152:in `respond'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:150:in `each'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:150:in `respond'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:107:in `respond_to'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in `perform_action_without_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:697:in `call_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:689:in `perform_action_without_benchmark'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
e:/ruby/lib/ruby/1.8/benchmark.rb:293:in `measure'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/rescue.rb:199:in `perform_action_without_caching'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:678:in `perform_action'
e:/ruby/lib/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/connection_adapters/abstract/query_cache.rb:33:in `cache'
e:/ruby/lib/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/query_cache.rb:8:in `cache'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:677:in `perform_action'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in `process_without_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:685:in `process_without_session_management_support'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/session_management.rb:123:in `process'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:388:in `process'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:171:in `handle_request'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:115:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:126:in `dispatch_cgi'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:9:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:112:in `handle_dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:78:in `service'
e:/ruby/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
e:/ruby/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
e:/ruby/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
e:/ruby/lib/ruby/1.8/webrick/server.rb:162:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
e:/ruby/lib/ruby/1.8/webrick/server.rb:95:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:92:in `each'
e:/ruby/lib/ruby/1.8/webrick/server.rb:92:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:23:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:82:in `start'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:62:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/commands/servers/webrick.rb:66
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:342:in `new_constants_in'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/commands/server.rb:39
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
script/server:3
app/views/books/index.html.erb:15:in `_run_erb_47app47views47books47index46html46erb'
app/views/books/index.html.erb:11:in `each'
app/views/books/index.html.erb:11:in `_run_erb_47app47views47books47index46html46erb'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:637:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:637:in `compile_and_render_template'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:365:in `render_template'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:316:in `render_file'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1100:in `render_for_file'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:858:in `render_with_no_layout'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:872:in `render_with_no_layout'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/layout.rb:262:in `render_without_benchmark'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
e:/ruby/lib/ruby/1.8/benchmark.rb:293:in `measure'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in `render'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:131:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:131:in `custom'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:152:in `call'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:152:in `respond'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:150:in `each'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:150:in `respond'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/mime_responds.rb:107:in `respond_to'
app/controllers/books_controller.rb:7:in `index'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1158:in `perform_action_without_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:697:in `call_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:689:in `perform_action_without_benchmark'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
e:/ruby/lib/ruby/1.8/benchmark.rb:293:in `measure'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/rescue.rb:199:in `perform_action_without_caching'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:678:in `perform_action'
e:/ruby/lib/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/connection_adapters/abstract/query_cache.rb:33:in `cache'
e:/ruby/lib/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/query_cache.rb:8:in `cache'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:677:in `perform_action'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in `send'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in `process_without_filters'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:685:in `process_without_session_management_support'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/session_management.rb:123:in `process'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:388:in `process'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:171:in `handle_request'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:115:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:126:in `dispatch_cgi'
e:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:9:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:112:in `handle_dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:78:in `service'
e:/ruby/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
e:/ruby/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
e:/ruby/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
e:/ruby/lib/ruby/1.8/webrick/server.rb:162:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
e:/ruby/lib/ruby/1.8/webrick/server.rb:95:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:92:in `each'
e:/ruby/lib/ruby/1.8/webrick/server.rb:92:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:23:in `start'
e:/ruby/lib/ruby/1.8/webrick/server.rb:82:in `start'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/webrick_server.rb:62:in `dispatch'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/commands/servers/webrick.rb:66
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:342:in `new_constants_in'
e:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.0.2/lib/active_support/dependencies.rb:496:in `require'
e:/ruby/lib/ruby/gems/1.8/gems/rails-2.0.2/lib/commands/server.rb:39
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `gem_original_require'
e:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in `require'
script/server:3
Request
Parameters:

None

Show session dump

---
:csrf_id: 4d7ca8c77d9629fbc9bbcac4d9848d21
flash: !map:ActionController::Flash::FlashHash {}

The error occurred while evaluating nil.name
是说这个book没有category.所以报的错的是 nil.name
而不是book.category

如果是关联有问题，应该报 undefined method 'category' for #<Book:xxxx>

谢谢CaiDeHen ：
问题已经解决，原来字段值为空的时候，调用一样会抛出异常。一个小小的问题，困扰了我一下午。再次感谢CaiDeHen。