如何使用mptt访问ManyRelatedManager对象属性

What I got:

我得到了什么:

class Category(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    slug = models.SlugField(unique=True)
    full_slug = models.CharField(max_length=256, null=True, blank=True)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)

class Work(models.Model):
    title = models.CharField(max_length=128, unique=True)
    slug = models.SlugField(max_length=128, unique=True)    
    category = TreeManyToManyField(Category, verbose_name='parent category', related_name='works')
    url = models.CharField(max_length=256, null=True, blank=True)

    def save(self, *args, **kwargs):
        if self.category:
            self.url = '%s/%s' % (self.category.full_slug, self.slug)
        else:
            self.url = '%s' % self.slug
        super(Work, self).save(*args, **kwargs)
class Category(MPTTMode