Python - SQLAlchemy之连表操作

ORM的两种创建方式

数据库优先：指的是先创建数据库，包括表和字段的建立，然后根据数据库生成ORM的代码，它是先创建数据库，再创建相关程序代码

代码优先：就是先写代码，然后根据代码去生成数据库结构。

代码优先创建数据库的本质：拿到类-->转换成table对象，然后根据table对象生成sql语句--> 生成数据库表结构

另外两个知识点：

改变数据输出的方式:可以在表的类中定义一个特殊成员:__repr__,return一个自定义的由字符串拼接的数据连接方式.
数据库中表关系之间除了MySQL中标准的外键(ForeignKey)之外,还可以创建一个虚拟的关系,比如group = relationship("Group",backref='uuu'),一般此虚拟关系与foreignkey一起使用.

SQLalchemy联表操作

1.一对多关系

需求:

用户组,有dba,ddd组
用户,用户只能属于一个用户组

两个表如下：

group表：

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user表：

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创建上述两个表：

#!/usr/bin/env python

# -*- coding: utf-8 -*-

# auth : pangguoping

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:123@192.168.29.128:3306/s13", max_overflow=5)

Base = declarative_base()

#创建一对多表

class Group(Base):

    __tablename__ = 'group'

    nid = Column(Integer,primary_key=True,autoincrement=True)

    caption = Column(String(32))

class User(Base):

    __tablename__ = 'user'

    nid = Column(Integer,primary_key=True,autoincrement=True)

    username = Column(String(32))

    group_id = Column(Integer,ForeignKey('group.nid'))

    group = relationship("Group",backref='uuu')

    #这个方法输出什么，对象就获取什么

    def __repr__(self):

        temp = '%s - %s:%s' %(self.nid,self.username,self.group_id)

        return temp

def init_db():

    Base.metadata.create_all(engine)

init_db()

向上述两个表中添加数据

Session = sessionmaker(bind=engine)

session = Session()

#向group表中添加数据

session.add(Group(caption='dba'))

session.add(Group(caption='sa'))

session.commit()

#向user表中添加数据

session.add_all([

    User(username='user1',group_id=1),

    User(username='user2',group_id=1),

    User(username='user3', group_id=2),

    User(username='user4', group_id=2)

]

)

session.commit()

查询user表中的所有用户：

# #以left.join查看

ret = session.query(User).join(Group,isouter=True).all()

print(ret)

sql = session.query(User).join(Group,isouter=True)

print(sql)

out:

[1 - user1:1, 2 - user2:1, 3 - user3:2, 4 - user4:2]
SELECT "user".nid AS user_nid, "user".username AS user_username, "user".group_id AS user_group_id
FROM "user" LEFT OUTER JOIN "group" ON "group".nid = "user".group_id

查询每个用户对应的组：

常规查询方式：

# #映射方式

sql = session.query(User.username,Group.caption).join(Group,isouter=True)

print(sql)

ret = session.query(User.username,Group.caption).join(Group,isouter=True).all()

print(ret)

out:

SELECT "user".username AS user_username, "group".caption AS group_caption
FROM "user" LEFT OUTER JOIN "group" ON "group".nid = "user".group_id
[('user1', 'dba'), ('user2', 'dba'), ('user3', 'sa'), ('user4', 'sa')]