class Parent(db.Model):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    name = Column(String(20))
 
 
class Child(db.Model):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    name = Column(String(20))
 
    parent_id = Column(Integer, ForeignKey('parent.id'))
    parent = relationship("Parent", backref=backref("child"))
 
    ###  只删除父级，子不影响
    # 1. parent_id = Column(Integer, ForeignKey('parent.id', ondelete="CASCADE"))
    #    parent = relationship("Parent", backref=backref("child", passive_deletes=True))
 
    ###  子级跟随删除
    # 2. parent = relationship("Parent", backref = backref("child", cascade="all, delete-orphan"))
    # 3. parent = relationship("Parent", backref = backref("child", cascade="all,delete"))
 
    ##  父级删除，子级不删除，外键更新为 null