title: Realm: sorting by property in child object
body: <p>My Show object looks like this:</p>
  
  <pre><code>class Show: RLMObject {
      dynamic var venue: Venue?
  }
  </code></pre>
  
  <p>and my Venue object:</p>
  
  <pre><code>class Venue: RLMObject {
      dynamic var title = ""
  }
  </code></pre>
  
  <p>I need to be able to sort my Show objects by their Venue object's titles. I tried the following but got an error:</p>
  
  <pre><code>allShowsByLocation = Show.allObjects().sortedResultsUsingProperty("venue.title", ascending: true)
  </code></pre>
  
  <p>The error is: Invalid sort column', reason: 'Column named '(null)' not found.</p>