title: Mongoose sort elements by counting numbers
body: <p>I have a documents like</p>
  
  <pre class="lang-js prettyprint-override"><code>{
    data: [{"channel":"712064846325219432","message":1019},{"channel":"712064884812021801","message":4}],
    user: '290494169783205888',
  },
  {
    data: [{"channel":"712064846325219432","message":2000},{"channel":"712064884812021801","message":500}],
    user: '534099893979971584',
  }
  </code></pre>
  
  <p>So how can I count <code>data</code>'s message and sort this documents by descending message?</p>