桥接关系表的Sqlalchemy事件

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

Base = declarative_base()

class EntityA(Base):
    __tablename__ = 'entity_a'
    id = Column(Integer, primary_key=True)
    entity_bs = relationship('EntityB', secondary='bridge_table')

class EntityB(Base):
    __tablename__ = 'entity_b'
    id = Column(Integer, primary_key=True)

class BridgeTable(Base):
    __tablename__ = 'bridge_table'
    entity_a_id = Column(Integer, ForeignKey('entity_a.id'), primary_key=True)
    entity_b_id = Column(Integer, ForeignKey('entity_b.id'), primary_key=True)