将order_with_respect_to设置为django orderable中的外键的外键

My models:

我的模特：

Class A(Orderable, Slugged):
   a = models.CharField(max_length=250, null=True, blank=True)
   class Meta:
        ordering = ("_order",)

Class B(Orderable, Slugged):
   a = models.ForeignKey(A, null=True, blank=True)
   b = models.CharField(max_length=250, null=True, blank=True)
   class Meta:
        ordering = ("_order",)
        order_with_respect_to = "a"

Class C(Orderable, Slugged):
   b = models.ForeignKey(B, null=True, blank=True)
   c = models.CharField(max_length=250, null=True, blank=True)
   class Meta:
        ordering = ("_order",)
        order_with_respect_to = "b__a"
Class A(Orderable, Slugge