title: Use result from previous query in another query in mongodb
body: <p>1st query:</p>
  
  <pre><code>    db.a.find({
      "type": ObjectId("50ed90f5a70defef23000002"),
      "config.name": "alpha"
   }, {
      "user": 1
   })
  </code></pre>
  
  <p>1st query result:</p>
  
  <pre><code>{ "_id" : ObjectId("5684035c91b4693b39547591"), "user" : ObjectId("54ba80ac2735eea200623612") }
  </code></pre>
  
  <p>2st query:</p>
  
  <pre><code>   db.users.find({
      "_id": ObjectId("54ba80ac2735eea200623612")
  }, {
      "lastname": 1
  })
  </code></pre>
  
  <p>can I combine these two queries into one query? 2st query uses 1st query result's <code>"user"</code> value <code>ObjectId("54ba80ac2735eea200623612")</code></p>