关于删除级联 - SQL Alchemy + Postgres

Question

我正在尝试在父（Product）和子表（ProductVariant）之间设置删除级联，但它无法正常工作。我做错了什么？

from sqlalchemy.orm import relationship, backref

Base = declarative_base()

class Product(Base):
    __tablename__ = 'product'
    id = Column(Integer, primary_key=True)
    title = Column(String)
    collection_id = Column(Integer, ForeignKey('collection.id'))


class ProductVariant(Base):
    __tablename__ = 'product_variant'
    id = Column(Integer, primary_key=True)
    title = Column(String)
    product_id = Column(Integer, ForeignKey(Product.id, ondelete='CASCADE'), nullable=False)
    product = relationship(Product, backref= backref("variants", cascade='delete'))