parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
ancestor = models.ManyToManyField('self', blank=True, editable=False, related_name='descendant', symmetrical=False)
parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
ancestor = models.ManyToManyField('self', blank=True, editable=False, related_name='descendant', symmetrical=False)