title: How to route new action with params in rails 3?
body: <p>I have 2 models: <code>User</code> and <code>Post</code><br>
  <code>User</code> has many <code>posts</code><br>
  <code>Post</code> belongs to <code>User</code></p>
  
  <p>Then I created a new action named <code>list</code> in <code>PostsController</code>, it takes user's <code>id</code> as params to find and list all posts posted by certain user.</p>
  
  <p>The URL is like <code>posts/list/:user_id</code></p>
  
  <p>How can I modify routes to perform this task?</p>