title: native reverse links in mongodb
body: <p>For example I created post with two comments:</p>
  
  <pre><code>db.posts.save(id: 'xxx');
  db.comments.save(id:'c1', post: 'xxx', text: 'first comment');
  db.comments.save(id:'c2', post: 'xxx', text: 'second comment');
  </code></pre>
  
  <p>I want to get data about comments in posts.find(): </p>
  
  <pre><code>[{id: 'xxx', commentsCount: 2}]
  </code></pre>
  
  <p>or better:</p>
  
  <pre><code>[{id: 'xxx', comments:['c1', 'c2']}]
  </code></pre>
  
  <p>I want to do it with native mongodb methods (without manual updating of collection 'posts'). Read about DbRefs and MapReduce, but didnt understand how to solve my task. </p>