sqlalchemy操作----多表关联

有二张表，一张作者表，一张书表，一个作者写多本书，一本书可以由多个作者写，与是通过新加一张关系表把他们联系起来

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# author aliex-hrg

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

engine = create_engine("mysql+pymysql://hrg:123@192.168.80.100:3306/test?charset=utf8")
Base = declarative_base()  #生成orm基类

Base = declarative_base()
book_to_author = Table('book_to_author', Base.metadata,
                        Column('book_id',Integer,ForeignKey('books.id')),
                        Column('author_id',Integer,ForeignKey('authors.id')),
                        )

class Author(Base):
    __tablename__ = 'authors' #表名
    id = Column(Integer, primary_key=True)
    name = Column(String(32))
    def __repr__(self):
        return self.name

class Book(Base):
    __tablename__ = 'books' #表名
    id = Column(Integer, primary_key=True)
    name = Column(String(32))
    authors = relationship('Author', secondary=book_to_author, backref='books')
    def __repr__(self):
        return self.name

Base.metadata.create_all(engine)  #创建表

a1 = Author(name='alex')
a2 = Author(name='keke')
a3 = Author(name='tom')
b1 = Book(name='linux')
b2 = Book(name='python')
b3 = Book(name='C++')

b1.authors = [a1,a2,a3]
b2.authors = [a1,a3]
b3.authors = [a3]

Session_class = sessionmaker(bind=engine)
Session = Session_class()

Session.add_all([a1,a2,a3,b1,b2,b3])
Session.commit()
print('--------通过书表查关联的作者---------')
book_obj = Session.query(Book).filter(Book.name=='linux').first()
print(book_obj.name,book_obj.authors)
print('--------通过作者表查关联的书---------')
author_obj = Session.query(Author).filter(Author.name=='alex').first()
print(author_obj.name,author_obj.books)
Session.commit()

#通过书删除作者  只会从book_to_author表中删除记录
sobj = Session.query(Author).filter(Author.name=='alex').first() #找出要删除作者alex的对象名
bobj = Session.query(Book).filter(Book.name=='linux').first()   #找出从linux这本书中删除
bobj.authors.remove(sobj)
Session.commit()

#直接删除作者　  会从authors,book_to_author中删除
#删除作者时，会把这个作者跟所有书的关联关系数据也自动删除
author_obj =Session.query(Author).filter_by(name="tom").first()
Session.delete(author_obj)
Session.commit()

　　。。。。