title: How to join query in mongodb?
body: <p>I have user document collection like this:</p>
  
  <pre><code>User {
     id:"001"
     name:"John",
     age:30,
     friends:["userId1","userId2","userId3"....]
  }
  </code></pre>
  
  <p>A user has many friends, I have the following query in SQL:</p>
  
  <pre><code>select * from user where in (select friends from user where id=?) order by age
  </code></pre>
  
  <p>I would like to have something similar in MongoDB.</p>