title: Rails routes custom nesting syntax
body: <p>I would like to map the following legacy url:</p>
  
  <blockquote>
    <p>/:user_id/comments</p>
  </blockquote>
  
  <p>comments is a resource, so in an idea world I would use something like:</p>
  
  <pre><code>resources :users, :prefix =&gt; nil do
    resources :comments
  end
  </code></pre>
  
  <p>Here I'm using prefix => nil but what I really want is to map just the user id. So instead of /users/:user_id/comments/ I have /:user_id/comments/.</p>
  
  <p>Note that I'm not interested in declaring a users resource, so if there's something like a namespace that I can use instead all the better.</p>