对象没有属性_set'，Django

I have these models:

我有这些模型:

class BlogCategory(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    def get_number_of_categorys_items(self):
        return self.post_set.count()


class Post(models.Model):
    title = models.CharField(max_length=130)
    content = models.TextField()
    category = models.ForeignKey(BlogCategory, related_name='blog_category')

    def __str__(self):
        return self.title
class BlogCate