问Django数据库迁移问题-缺少“迁移类”错误

EN

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_api_serializers(ModelSerializer):
class Meta:
    model = Dag
    fields = ('dag_id','is_paused','is_active','is_subdag','last_scheduler_run','last_pickled','last_expired','scheduler_lock','pickle_id','fileloc','owners')

    Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "C:\env\lib\site-packages\django\core\management\__init__.py", line 363, in execute_from_command_line
    utility.execute()
  File "C:\env\lib\site-packages\django\core\management\__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\env\lib\site-packages\django\core\management\base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\env\lib\site-packages\django\core\management\base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "C:\env\lib\site-packages\django\core\management\commands\makemigrations.py", line 96, in handle
    loader = MigrationLoader(None, ignore_no_migrations=True)
  File "C:\env\lib\site-packages\django\db\migrations\loader.py", line 52, in __init__
    self.build_graph()
  File "C:\env\lib\site-packages\django\db\migrations\loader.py", line 203, in build_graph
    self.load_disk()
  File "C:\env\lib\site-packages\django\db\migrations\loader.py", line 117, in load_disk
    "Migration %s in app %s has no Migration class" % (migration_name, app_config.label)
django.db.migrations.exceptions.BadMigrationError: Migration serializers in app snippets has no Migration class