title: Storing timestamp in joining node value instead of Boolean in Firebase database
body: <p>Say that I have node <code>user</code>, <code>item</code> and <code>user_items</code> used to join them.</p>
  
  <p>Typically one would<code>(as advised in official documents and videos)</code> use such a structure:</p>
  
  <pre><code> "user_items": {
      "$userKey": {
        "$itemKey1": true,
        "$itemKey2": true,
        "$itemKey3": true
      }
    }
  </code></pre>
  
  <p>I would like to use the following structure instead:</p>
  
  <pre><code> "user_items": {
      "$userKey": {
        "$itemKey1": 1494912826601,
        "$itemKey2": 1494912826602,
        "$itemKey3": 1494912826603
      }
    }
  </code></pre>
  
  <p>with values being a timestamp value. So that i can order them by creation date also while being able to tell the associated time. Seems like killing two birds with one stone situation. Or is it?</p>
  
  <p>Any down sides to this approach?</p>
  
  <p>EDIT: Also I'm using this approach for the boolean fields such as: <code>approved_at</code>, <code>seen_at</code>,... etc instead of using two fields like:</p>
  
  <pre><code>"some_message": {
    "is_seen": true,
    "seen_timestamp": 1494912826602,
  }
  </code></pre>