Flask SQL-Alchemy创建层次表到同一表的多个外键

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

class Category(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    parent_id = db.Column(db.Integer, db.ForeignKey('category.id'))
    children = db.relationship('Category', backref=db.backref('parent', remote_side=[id]))

    def __repr__(self):
        return self.name

root_category = Category(name='Root')
db.session.add(root_category)
db.session.commit()

child_category = Category(name='Child', parent=root_category)
db.session.add(child_category)
db.session.commit()