static $has_one = array(
      'TickerImage' => 'Image'
   );
   static $summary_fields = array(
      'TickerImage.CMSThumbnail.Tag'
   );

class Competition extends DataObject
{
   
   public static $db = array(
      'Name' => 'Varchar(255)',
      'Start' => 'SSDatetime',
      'End' => 'SSDatetime'
   );
   
   public static $summary_fields = array(
      'Name',
      'Duration'
   );
   
   public function Duration()
   {
      return $this->dbObject('Start')->Full() . ' - ' . $this->dbObject('End')->Full();
   }
   
}

Error: "Uncaught Exception: Unable to traverse to related object field [Duration] on [Competition]" at line 2492 of /sapphire/core/model/DataObject.php

public static $searchable_fields = array(
   'Name'
);