python的ORM技巧记录

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

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey, Index
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker
from django.db import models

Base = declarative_base()

class Hobby(Base):
    __tablename__ = 'hobby'
    id = Column(Integer, primary_key = True)
    caption = Column(String(50), default = '篮球')

class Person(Base):
    __tablename__ = 'Person'
    nid = Column(Integer, primary_key = True)
    name = Column(String(32), index = True, nullable = True)
    hobby_id = Column(Integer, ForeignKey("hobby.id"))

    # hobby属性不管有没有数据库都不会生成这个字段，该字段只作为属性正向和反向关联Hobby
    hobby = relationship("Hobby", backref = 'pers')

# 创建连接池,pool_size限制连接数10个,max_overflow设置最多多2个连接，也就是最多12个连接。
engine = create_engine("mysql+pymysql://username:[email protected]:3306/testdb?charset=utf8", max_overflow=2, pool_size=10)
Session = sessionmaker(bind = engine)
# 从连接池中获取连接
session = Session()

# 查询一，直接查询表Person的数据
obj1 = session.query(models.Person).all()
for row in obj1:
    print(row.name, row.hobby_id)

# 查询二，Person和Hobby的关联查询，使用join方式
obj2 = session.query(models.Person.name, models.Hobby.caption).join(models.Hobby, isouter = True).all()
for row in obj2:
    print(row[0].name, row[1].caption)

# 查询三，Person和Hobby的关联查询，使用relationship方式，正向关联
obj3 = session.query(models.Person).all()
for row in obj3:
    print(row.name, row.hobby.caption)

# 查询四，查询喜欢篮球的所有人，使用relationship方式，反向向关联
obj4 = session.query(models.Hobby).filter(models.Hobby.id == 2).first()
persons = obj4.pers
for row in persons:
    print(row.name)

# 关闭数据库连接
session.close()