SQLAlchemy删除父项既不删除子项也不设置子ForeignKey值为空

from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    children = relationship("Child", back_populates="parent")

class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", back_populates="children")

from sqlalchemy import ForeignKeyConstraint

ForeignKeyConstraint(['id'], ['child.parent_id'], ondelete='SET NULL')