Ejemplo de Backref en los modelos

models.py

from flask_sqlalchemy import SQLAlchemy


db = SQLAlchemy()


class Users(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(50), unique=True, nullable=False)
    comments = db.relationship('Comments', backref='author', lazy=True)


    def __repr__(self):
        return f'<User {self.username}>'


    def serialize(self):
        return {
            "id": self.id,
            "username": self.username
        }

class Comments(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    text = db.Column(db.String(500), nullable=False)
    user_id = db.Column(db.Integer, db.ForeignKey('users.id'), nullable=False)

    def __repr__(self):
        return f'<Comment: {self.id} - User: {self.user_id}>'

    def serialize(self):
        return {
            "id": self.id,
            "user_id": self.user_id,
            "text": self.text
        }