Python SQLAlchemy之多对多

1.新建三张表host,host_user,host_to_host_user

host:

host_user:

host_to_host_user:

第一个问题：我们需要查询c1服务器里面的用户都有哪些？

如果使用传统的方法，我们使用下面的语句来实现功能：

#取主机名为c1的服务器ID号
host_obj = session.query(Host).filter(Host.hostname == ‘c1‘).first()
#通过获取到的服务器ID号去对应的关系表中找到用户ID，得到一个列表，里面是元祖
host_2_host_user = session.query(HostToHostUser.host_user_id).filter(HostToHostUser.host_id == host_obj.nid).all()
# print (host_2_host_user)   #[(1,), (2,), (3,)]
#处理元祖
ret = zip(*host_2_host_user)
# print (list(ret)[0])
users = session.query(HostUser.username).filter(HostUser.nid.in_(list(ret)[0])).all()
print (users)执行结果：

[(‘root‘,), (‘db‘,), (‘nb‘,)]

使用新方式来实现，在建表的步骤里面我们加入了新的relationship:

#coding:utf-8
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey, UniqueConstraint, Index
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy import create_engine

engine = create_engine("mysql+pymysql://root:[email protected]/beadata", max_overflow=5)

Base = declarative_base()

class Host(Base):
    __tablename__ = ‘host‘
    nid = Column(Integer,primary_key=True,autoincrement=True)
    hostname = Column(String(32))

class HostUser(Base):
    __tablename__ = ‘host_user‘
    nid = Column(Integer,primary_key=True,autoincrement=True)
    username = Column(String(32))

class HostToHostUser(Base):
    __tablename__ = ‘host_to_host_user‘
    nid = Column(Integer,primary_key=True,autoincrement=True)
    host_id = Column(Integer,ForeignKey(‘host.nid‘))
    host_user_id = Column(Integer,ForeignKey(‘host_user.nid‘))

    host = relationship(‘Host‘,backref = ‘h‘)
    host_user = relationship(‘HostUser‘,backref = ‘u‘)

Session = sessionmaker(bind=engine)
session = Session()

# session.add_all([
#     Host(hostname=‘c1‘),
#     Host(hostname=‘c2‘),
#     Host(hostname=‘c3‘),
#     Host(hostname=‘c4‘),
#     Host(hostname=‘c5‘),
# ])
#
# session.add_all([
#     HostUser(username=‘root‘),
#     HostUser(username=‘db‘),
#     HostUser(username=‘nb‘),
#     HostUser(username=‘sb‘),
# ])
#
# session.add_all([
#     HostToHostUser(host_id=‘1‘,host_user_id=‘1‘),
#     HostToHostUser(host_id=‘1‘,host_user_id=‘2‘),
#     HostToHostUser(host_id=‘1‘,host_user_id=‘3‘),
#     HostToHostUser(host_id=‘2‘,host_user_id=‘2‘),
#     HostToHostUser(host_id=‘2‘,host_user_id=‘4‘),
#     HostToHostUser(host_id=‘2‘,host_user_id=‘3‘),
# ])

host_obj = session.query(Host).filter(Host.hostname == ‘c1‘).first()
print (host_obj.nid)
print (host_obj.hostname)
print (host_obj.h)执行结果：

1
c1
[<__main__.HostToHostUser object at 0x0000000003FA3208>, <__main__.HostToHostUser object at 0x0000000003FA3278>, <__main__.HostToHostUser object at 0x0000000003FA32E8>]

host_obj.h获取到三个对象，然后循环这三个对象，可以得到这三个对象中的host_to_host_user表中的host_user.nid,host_user.host_id,host_user.host_user_id:

for item in host_obj.h:
    print (item.host_user,item.host_user.username)

执行结果：

1
c1
[<__main__.HostToHostUser object at 0x0000000003FA2208>, <__main__.HostToHostUser object at 0x0000000003FA2278>, <__main__.HostToHostUser object at 0x0000000003FA22E8>]
<__main__.HostUser object at 0x0000000003FA28D0> root
<__main__.HostUser object at 0x0000000003FA2A90> db
<__main__.HostUser object at 0x0000000003FA2C50> nb