title: Firebase data sorting by timestamp
body: <p>I have a Firebase node structure like the following.</p>
  
  <pre><code>categories
      category1
          timestamps
              23c491sdfwrasddgasd: true
              asdf8234dfjalskdask: true
              39fsdizxcasfgbskdf7: true
              2asqwizxov340bs9fms: true
      category2
          timestamps
              p23491sdfwrasddgasd: true
              asdfpovaskdrfskdask: true
      category3
          timestamps
              vmk339ffk3rasddgasd: true
              asdf8234dfjalskdvml: true
              bnwdwizxcasfgbskdf7: true
  </code></pre>
  
  <p>The timestamp keys are the keys generated by <code>push()</code>. I need to sort the categories by the oldest timestamp key they have. Only the oldest timestamp from the timestamps node will count.</p>
  
  <p>Basically I need to sort the categories by the time they were modified for the first time.</p>
  
  <p>Any help is welcome including tips to redesign the node.</p>