title: mongoose middleware pre update
body: <p>I am using </p>
  
  <pre><code>schema.pre('save', function (next) {
    if (this.isModified('field')) {
      //do something
    }
  });
  </code></pre>
  
  <p>but I now need to use this same function <code>isModified</code> in a <code>schema.pre('update'</code> hook, but it does not exists. Does anyone know how I can use this same functionality in the <code>update</code> hook?</p>