title: mongoose pre update not firing
body: <p>I have follow the directions in mongoose <a href="http://mongoosejs.com/docs/middleware.html#notes" rel="noreferrer">here</a></p>
  
  <pre><code>PostSchema.pre('update', function() {
      console.log('pre update');
      console.log(this);
  });
  </code></pre>
  
  <p>it is not firing this middleware. Am I missing something here?</p>
  
  <p>I have added <code>next</code> so it looks exactly like my pre save, however that still does nothing.</p>