public function getDelivery($deliveryResultIdentifier)
    {
        $sql = 'SELECT ' . self::DELIVERY_COLUMN . ' FROM ' . self::RESULTS_TABLENAME . ' WHERE ' . self::RESULTS_TABLE_ID . ' = ?';
        $params = array($deliveryResultIdentifier);
        return $this->getPersistence()->query($sql, $params)->fetchColumn();
    }