Простое дерево в админке Джанго

class Bus(MPTTModel):
	name = models.CharField(max_length=100)

class Device(Category):
	name = models.CharField(max_length=100)
	bus = models.ForeignKey(Bus)

class Tag(models.Model):
	title = models.CharField(max_length=200)
	device = models.ForeignKey(Device)