SQLAlchemy将多个子类合并到一个关系中

from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    children = relationship("Child", back_populates="parent", cascade="all, delete-orphan", single_parent=True, lazy="joined")

    __mapper_args__ = {
        'polymorphic_identity': 'parent',
        'polymorphic_on': name
    }

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

    __mapper_args__ = {
        'polymorphic_identity': 'child'
    }

engine = create_engine('sqlite:///example.db')
Base.metadata.create_all(engine)