public function getMigrationBatches(): array
    {
        $stmt = $this->pdo->prepare("select * from {$this->table} order by batch, migration");
        $stmt->execute();

        $array = [];
        foreach ($stmt->fetchAll(PDO::FETCH_ASSOC) as $item) {
            $array[$item['migration']] = $item['batch'];
        }

        return $array;
    }