Python SQLAlchemy多对多外键关联时表结构

 # 创建多对多表结构
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.engine import create_engine
 from sqlalchemy import INTEGER, Column, CHAR, ForeignKey
 from sqlalchemy.orm import relationship
 Base = declarative_base()
 class Book(Base):
     __tablename__ = 'book'
     id = Column(INTEGER, primary_key=True)
     name = Column(CHAR(20), nullable=False)
     b2t = relationship('Tag', backref='t2b', secondary='booktag')
 
     def __str__(self):
         return self.name
 
 class Tag(Base):
     __tablename__ = 'tag'
     id = Column(INTEGER, primary_key=True)
     name = Column(CHAR(20), nullable=False)
 
     def __str__(self):
         return self.name
 
 class BookTag(Base):
     __tablename__ = 'booktag'
     id = Column(INTEGER, primary_key=True)
     # ondelete、onupdate设置为同步更新、同步删除，代码中对应的值为CASCADE，应注意，否则在进行数据删除时将会报错。
     book_id = Column(INTEGER, ForeignKey('book.id', ondelete='CASCADE', onupdate='CASCADE'))
     tag = Column(INTEGER, ForeignKey('tag.id', ondelete='CASCADE', onupdate='CASCADE'))
 
 engine = create_engine("mysql+mysqlconnector://root:@127.0.0.1:3306/sqlalchemy?charset=utf8")
 if __name__ == '__main__':
     Base.metadata.create_all(engine)