title: How to query data by using children attribute?
body: <p>I have following code:</p>
  
  <pre><code>Model.Class.find().populate({
      path: 'student',
      select: '-_id grade',
  }).where("student.grade").equals("80").exec(function (err, result) {
      log.info(result);
  });
  </code></pre>
  
  <p>It will give me empty array. </p>
  
  <p>How to query data by using children attribute? Is there something wrong inside my code?</p>