title: Using positional operator with two-level hierarchies in Mongo
body: <p>I have a hierarchy of, let say, <code>posts -&gt; comments -&gt; votes</code>.  </p>
  
  <p>How do I update a certain vote on certain comment?<br>
  For comment it is <code>comments.$</code>, but I obviously can't do <code>comments.$.votes.$</code>.</p>