django动态生成模型选择

Question

models.py

class Make(models.Model):
    name = models.CharField(max_length = 50)
    #category = models.ManyToManyField(Category)
    def __unicode__(self):
        return self.name


class Category(models.Model):
    make = models.ManyToManyField(Make)
    name = models.CharField(max_length = 40)
    def __unicode__(self):
        return self.name

class Subcategory(models.Model):
    name = models.CharField(max_length = 40)
    category = models.ForeignKey(Category)
    def __unicode__(self):
        return self.name

makes = Make.objects.all()
MAKE_CHOICES = [(str(cat.name),str(cat.name)) for cat in makes]
categories = Category.objects.all()
CAT_CHOICES = [(str(cat.name),str(cat.name)) for cat in categories]
subcategories = Subcategory.objects.all()
SUB_CHOICES = [(str(cat.name),str(cat.name)) for cat in subcategories]
class Subscription(models.Model):
    make = models.CharField(max_length = 40,choices = MAKE_CHOICES)
    #category = models.ManyToManyField(Category)

    category = models.CharField(max_length = 40,choices = CAT_CHOICES)
    subcategory = models.CharField(max_length = 40,choices = SUB_CHOICES)
    #subcategory = models.ManyToManyField(Subcategory)
    fixer = models.ForeignKey(Fixer)
    subscribed_on = models.DateTimeField()
    expires_on = models.DateTimeField()
    user = models.ForeignKey(User)

疑问

当显示或提示订阅字段时，我需要它们动态生成， 如果选择了一个类别，那么子类别字段应该只显示与该特定字段相关的所有子类别。