问如何按类别添加自增字段

EN

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    # I tried this, but this did not work
    sub_id = db.Column(db.Integer, nullable=True, default=int(f'select count(*) from post where category_id={category_id}')+1)
    title = db.Column(db.String(120), unique=True, nullable=False)
    date_posted = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    content = db.Column(db.Text, nullable=False)
    # user in Foreignkey is table name
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    category_id = db.Column(db.Integer, db.ForeignKey('post_category.id'), nullable=False)

    def post_content_render_markdown(self):
        return markdown(self.content, extensions=[
                'markdown.extensions.extra',
                'markdown.extensions.codehilite',
            ])

    def __repr__(self):
        return f"Post('{self.title}', {self.category.name}, '{self.date_posted}')"

class PostCategory(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(120), unique=True, nullable=False)
    description = db.Column(db.Text, nullable=False)
    posts = db.relationship('Post', backref='category', lazy=True)

    def __repr__(self):
        return f"Post Category('{self.id}', '{self.name}')"

sub_id = db.Column(db.Integer, nullable=True, default=int(f'select count(*) from post where category_id={category_id}')+1)
NameError: name 'category_id' is not defined