protected function getKeys()
    {
        $identifier = $this->getIdentifier();

        $query = new Query();
        $selectNode = new Node\SelectNode([$identifier]);
        $query->setSelect($selectNode);
        $queryResult = $this->query($query);
        $keysArray = [];

        foreach ($queryResult as $row) {
            $keysArray[] = $row[$identifier];
        }

        return $keysArray;
    }