title: how iterate document with hash with associated values
body: <p>Example document:</p>
  
  <pre><code>{
      "_id" : ObjectId("53c32ac056c02c506b000003"),
      "participants" : {
          "user" : ObjectId("53c32ac056c02c506b000002"), # id of the user from User model
          "active" : "pending"
      }
  }
  </code></pre>
  
  <p>How do I show the user name using the ObjectId from the participants hash?</p>
  
  <p>I have tried many, belongs_to but nothing works :(</p>