接上一篇：https://www.jianshu.com/p/161205d5c4b7
乔巴.png
一对多和多对多最大的区别在于第三张表上，也就是说这两张表没有直接关系，需要第三张关系表来连接两张表，且SQLAlchemy虽然可以自动生成第三张表，但是它并不友好。因此不讨论，这里我们手动创建第三张表。
1 建立关系
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
from sqlalchemy import Column, Integer, String, Foreignkey
from sqlalchemy.orm import relationship
class Girls(Base):
__tablename__ = "girl"
id = Column(Integer,primary_key=True)
name = Column(String(32))
g2b = relationship("Boys",backref="b2g",secondary="hotel") # secondary作为中间桥梁（媒婆），关联上Boys对象，也就是这一篇幅的核心多对多关系就这样产生了
class Boys(Base):
__tablename__ = "boy"
name = Column(String(32))
class Hotel(Base):
__tablename__ = "hotel"
id = Column(Integer,primary_key=True)
boy_id = Column(Integer,Foreignkey("boy.id"))
girl_id = Column(Integer.Foreignkey("girl.id"))
from sqlalchemy import create_engine
engine = create_engine("mysql+pymysql://root:@127.0.0.1:3306/sqlalchemy?charset=utf8")
Base.metadata.create_all(engine)
2 ManyToMany__insert
from create_table_M2M import engine
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(engine)
db_session = Session()
from create_table_M2M import Girls,Boys
# 1 增加数据 - ralationship正向
girl_obj = Girls(name="Carrie")
girl_obj.g2b = [Boys(name="Gavin")] # 多对多的关系这里用列表
db_session.add(girl_obj)
db_session.commit()
# 1 增加数据 - ralationship反向
boy_obj = Boys(name="Tom")
boy_obj.b2g = [Girls(name="罗y风"),Girls(name="谢y琳")]
db_session.add(boy_obj)
db_session.commit()
3 ManyToMany_select
from create_table_M2M import engine
from sqlalchemy.orm import sessionmaker
Session = sessionmaker(engine)
db_session = Session()
from create_table_M2M import Girls,Boys
# 1 查询数据 - ralationship正向
girl_obj_list = db_session.query(Girls).all() # 注意all()返回的是一个列表
for girl in girl_obj_list:
for boy in girl.g2b： # 注意girl.g2b返回的是列表
print(girl.name, boy.name)
# 1 查询数据 - ralationship正向
boy_obj_list = db_session.query(Boy).all() # 注意all()返回的是一个列表
for boy in boy_obj_list:
for girl in boy.b2g： # boy.b2g返回的是列表
print(girl.name, boy.name)
4 ManyToMany_select_高级操作
未完待续。。。