Django数据库迁移问题 - 缺少“迁移类”错误

my models.py file,

我的models.py文件,

class Dag(models.Model):
    dag_id = models.CharField(primary_key=True, max_length=250)
    is_paused = models.IntegerField(blank=True, null=True)
    is_subdag = models.IntegerField(blank=True, null=True)
    is_active = models.IntegerField(blank=True, null=True)
    last_scheduler_run = models.DateTimeField(blank=True, null=True)
    last_pickled = models.DateTimeField(blank=True, null=True)
    last_expired = models.DateTimeField(blank=True, null=True)
    scheduler_lock = models.IntegerField(blank=True, null=True)
    pickle_id = models.IntegerField(blank=True, null=True)
    fileloc = models.CharField(blank=True, max_length=250)
    owners = models.CharField(blank=True, max_length=250)

    def _str_(self):
        return self.dag_id 

    class Meta:
        managed = True
        db_table = 'dag'
class Dag