知乎： 使用 sqlalchemy 的 orm 方式操作数据库是一种怎样的体验？
答： 酸爽！
本文基于：win10 + python3.4 + sqlAlchemy 1.0.13
先看一个图（来源）：
这是 sqlalchemy 的层级图。不难发现，其中 orm 是最*的封装。
ORM 基本操作步骤如下：
1. 建立连接
from sqlalchemy import create_engine
engine = create_engine('sqlite:///:memory:', echo=True)
2. 建立会话
from sqlalchemy.orm import Session session = Session(engine)
3. 声明基类
from sqlalchemy.ext.declarative import declarative_base Base = declarative_base()
4. 定义表（继承基类）
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship class Parent(Base):
__tablename__ = 'parent' id = Column(Integer, primary_key=True) children = relationship("Child") def __repr__(self):
return "<Parent(id='{}', children='{}')>".format(self.id, self.children) class Child(Base):
__tablename__ = 'child' id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id')) def __repr__(self):
return "<Child(id='{}', parent_id='{}')>".format(self.id, self.parent_id)
5. 生成表
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine) # 生产环境只需运行一次！！！！
6. 添加记录
session.add_all([Parent(children=[Child() for j in range(5)]) for i in range(2)])
session.commit()
7. 查询
# 懒加载 (lazyload)
# load everything, no eager loading.
for parent in session.query(Parent):
print(parent.children) # 联合加载 (joinedload)
# load everything, joined eager loading.
for parent in session.query(Parent).options(joinedload("children")):
parent.children # 子查询加载 (subqueryload)
# load everything, subquery eager loading.
for parent in session.query(Parent).options(subqueryload("children")):
parent.children
8. 完整代码
from sqlalchemy import create_engine
from sqlalchemy.orm import Session
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.orm import joinedload, subqueryload # 建立连接
engine = create_engine('sqlite:///:memory:', echo=True) # 建立会话
session = Session(engine) # 声明基类
Base = declarative_base() # 定义表（继承基类）
class Parent(Base):
__tablename__ = 'parent' id = Column(Integer, primary_key=True) children = relationship("Child") def __repr__(self):
return "<Parent(id='{}', children='{}')>".format(self.id, self.children) class Child(Base):
__tablename__ = 'child' id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey('parent.id')) def __repr__(self):
return "<Child(id='{}', parent_id='{}')>".format(self.id, self.parent_id) # 生成表
Base.metadata.drop_all(engine)
Base.metadata.create_all(engine) # 生产环境只需运行一次！！！！ # 添加记录
session.add_all([Parent(children=[Child() for j in range(5)]) for i in range(2)])
session.commit() # 查询
# lazyload
# load everything, no eager loading.
for parent in session.query(Parent):
print(parent.children) # joinedload
# load everything, joined eager loading.
for parent in session.query(Parent).options(joinedload("children")):
parent.children # subqueryload
# load everything, subquery eager loading.
for parent in session.query(Parent).options(subqueryload("children")):
parent.children