来自 Django书:

如果需要在尚未创建的模型上创建关系定义后，您可以使用模型的名称，而不是模型对象本身:

If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:

例如:

class MenuItem(model.Models)
    title = models.CharField(max_length=200)
    submenus = models.ManyToManyField('Menu', blank=True, null=True)
                                      ^    ^