public function getLowestFreeIndexNumber()
    {
        $query = $this->createQuery();
        $query->statement('SELECT index_number from ' . strtolower($this->objectType) . ' WHERE deleted=0');
        $index = 1;
        $results = $query->execute(true);
        if (false === is_array($results) || sizeof($results) < 1) {
            return $index;
        }
        $indexes = array();
        foreach ($results as $result) {
            $indexes[] = (integer)$result['index_number'];
        }
        for ($index = 1; $index <= sizeof($indexes) + 1; $index++) {
            if (false === in_array($index, $indexes)) {
                break;
            }
        }
        return $index;
    }