title: Why can't I run explain on MongoDB update?
body: <p>I'm trying to explain a mongoDB update; but nothing is happening? Does Mongo not support this?</p>
  
  <pre><code>&gt; db.movies.update({"actors.name": "Christian Bale"}, {$set: {"actors.$.name": "Christina Bale"}}, {$explain: 1});
  &gt;
  </code></pre>
  
  <p>I've tried other variations using <code>.explain()</code> and <code>._addSpecial("$explain", 1")</code>. Both of these produce the following error:</p>
  
  <blockquote>
    <p>Thu Aug  1 11:26:46.368 JavaScript execution failed: TypeError: Cannot call method 'explain' of undefined</p>
  </blockquote>