title: how to use swift 4 sorting method using #keyPath
body: <p>I have an array of nsmanaged object class. And i want to sort the array using particular element.</p>
  
  <p>every time i try to use it it gives me an error of <strong>Argument of '#keyPath' refers to non-'@objc' property 'questions'</strong> and <strong>Cannot convert value of type 'NSSortDescriptor' to expected element type 'ServiceQuestion'</strong></p>
  
  <pre><code>[NSSortDescriptor(key: #keyPath(objectClassArray), ascending: true)]
  </code></pre>