public function __construct($facetId, $value, $feed = null)
 {
     parent::__construct('cluster', $value, $feed);
     $this->facet_id = $facetId;
 }
 public function __construct($key, $sort, $feed = null)
 {
     parent::__construct($key, $sort, $feed);
     $this->sort_order = $sort;
 }