title: MongoDB sum of two nested field is less than some ratio of another nested field
body: <p>I've documents as follows:</p>
  
  <pre><code>{
     id: "14530529:V10AFRIN2018-AED15C",
     daily_log:{
        downloaded: 12345,
        uploaded:   10000,
        total:      50000
     }
  }
  </code></pre>
  
  <p>I need to find documents if <b>sum</b> of <b>daily_log.downloaded</b> and <b>daily_log.uploaded</b> is greater than 80% percentage of <b>daily_log.total</b></p>