sqlalchemy中的多对多自我指涉关系

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

Base = declarative_base()

association_table = Table('association', Base.metadata,
    Column('parent_id', Integer, ForeignKey('node.id')),
    Column('child_id', Integer, ForeignKey('node.id'))
)

class Node(Base):
    __tablename__ = 'node'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    children = relationship('Node',
                            secondary=association_table,
                            primaryjoin=id==association_table.c.parent_id,
                            secondaryjoin=id==association_table.c.child_id,
                            backref='parents')

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