code: 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;
      }
docs: Get the completed migrations with their batch numbers.
  
  @return array