四十：数据库之SQLAlchemy实现排序的三种方式

SQLAlchemy实现排序有三种方式
一：order_by：查询的时候使用此方式根据某个字段或模型下的属性进行排序
二：模型定义的时候，指定排序方式
三：一对多的时候，relationship的order_by参数

准备工作

from sqlalchemy import create_engine, Column, Integer, String, Float, Text, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref

# 数据库信息
host = '127.0.0.1'
port = '3306'
database = 'db_to_sqlalchemy'
username = 'root'
password = '123456'

# 数据库类型+连接数据库的插件，这里使用的pymysql
DB_URI = f'mysql+pymysql://{username}:{password}@{host}:{port}/{database}'

engine = create_engine(DB_URI)  # 创建引擎
Base = declarative_base(engine)  # 使用declarative_base创建基类
session = sessionmaker(engine)()

class Article(Base):
    __tablename__ = 'article'
    id = Column(Integer, primary_key=True, autoincrement=True)
    title = Column(String(50), nullable=False)
    create_time = Column(DateTime, nullable=False, default=datetime.now)

    def __repr__(self):
        return f'title: {self.title}、create_time: {self.create_time}'


Base.metadata.drop_all()  # 删除所有表
Base.metadata.create_all()  # 创建表

article1 = Article(title='title1')
session.add(article1)
session.commit()
time.sleep(1)
article2 = Article(title='title2')
session.add(article2)
session.commit()

一：order_by：查询的时候使用此方式根据某个字段或模型下的属性进行排序

默认升序

倒序：desc()

或者使用sqlalchemy.desc

二：模型定义的时候，指定排序方式

升序

倒序

class Article(Base):
    __tablename__ = 'article'
    id = Column(Integer, primary_key=True, autoincrement=True)
    title = Column(String(50), nullable=False)
    create_time = Column(DateTime, nullable=False, default=datetime.now)

    __mapper_args__ = {
        'order_by': create_time.desc()
    }

    def __repr__(self):
        return f'title: {self.title}、create_time: {self.create_time}'

三：一对多的时候，relationship的order_by参数

class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True, autoincrement=True)
    username = Column(String(50), nullable=False)


class Article(Base):
    __tablename__ = 'article'
    id = Column(Integer, primary_key=True, autoincrement=True)
    title = Column(String(50), nullable=False)
    create_time = Column(DateTime, nullable=False, default=datetime.now)
    uid = Column(Integer, ForeignKey('user.id'))
    author = relationship('User', backref=backref('article', order_by=create_time.desc()))

    # __mapper_args__ = {
    #     'order_by': create_time
    # }

    def __repr__(self):
        return f'title: {self.title}、create_time: {self.create_time}'

升序

若未在backref中指定排序规则，则会找__mapper_args__下的排序规则