rails map.resources和helper url

我有两个这样的模型:

class Topic < ActiveRecord::Base
  has_many :articles
end

class Article < ActiveRecord::Base
  belongs_to :user
  belongs_to :topic
  has_many :comments
end

我已经设置了资源映射,如下所示:

map.resources :topics do |topic|
  topic.resources :articles
end

当我调用适当的url(例如 /:topic_slug/articles/2 )在我的文章视图中,我使用partial来处理文章的创建和编辑,如下所示:

<% form_for(@article) do |f| %>
  ...
<% end %>

如果我试图创建一篇新文章或编辑一篇现有文章,则出现以下错误:

NoMethodError in Articles#new

Showing app/views/articles/_form.html.erb where line #1 raised:

undefined method `articles_path' for #<ActionView::Base:0x103d11dd0>
Extracted source (around line #1):

1: <% form_for(@article) do |f| %>
2:   <%= f.error_messages %>
3: 
4:   <p>
Trace of template inclusion: app/views/articles/new.html.erb

有人知道我做错了什么,我错过了什么吗?