数据查询懒加载技术

#encoding: utf-8

from sqlalchemy import create_engine,Column,Integer,String,Float,func,and_,or_,Text,
    ForeignKey,DateTime
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker,relationship,backref
from random import randint
from datetime import datetime

HOSTNAME = '127.0.0.1'

PORT = 3306

DATABASE = 'first_sqlalchemy'

USERNAME = 'root'

PASSWORD = '123456'

#dialect+driver://username:password@host:port/database
DB_URI = "mysql+pymysql://{username}:{password}@{host}:{port}/" 
         "{db}?charset=utf8".format(username=USERNAME,password=PASSWORD,host=HOSTNAME,port=PORT,db=DATABASE)

engine = create_engine(DB_URI)

Base = declarative_base(engine)

# Session = sessionmaker(engine)
# session = Session()

session = sessionmaker(engine)() #Session(**local_kw)


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer,primary_key=True,autoincrement=True)
    username = Column(String(32),nullable=False)

    # articles = relationship("Article") #获取用户发布所有文章


class Article(Base):
    __tablename__ = 'article'
    id = Column(Integer,primary_key=True,autoincrement=True)
    title = Column(String(50),nullable=False)
    content = Column(Text,nullable=False)
    uid = Column(Integer,ForeignKey('user.id',ondelete='RESTRICT'),nullable=False)

    #没有加载懒加载技术的relationship
    # author = relationship('User',backref='articles')
    #注意是user表对article表的懒加载，写在多关系一边
    author = relationship('User',backref=backref('article',lazy=
                                                 'dynamic'))

    def __repr__(self):
        return '<article:%s>'%self.title

s
# Base.metadata.drop_all()
#
# Base.metadata.create_all()
#
# user = User(username='zhiliao')
#
# for x in range(100):
#     article = Article(title='title%s'%x,content='content%s'%x)
#     article.author = user
#     session.add(article)
# session.commit()
#获取该用户的所有文章
# user = session.query(User).first()
#
# print(user.articles)

#如果获取该用户今天发表的文章
# articles = user.articles
# article_today = [article for article in articles if article.create_time.day=='today']
#如果想上面那样做的话，很耗费性能，等于我要全部查一遍


# user = session.query(User).first()
# print(type(user.articles)) #<class 'sqlalchemy.orm.collections.InstrumentedList'>,这是没有加载lazy

user = session.query(User).first()
print(type(user.articles))#<class 'sqlalchemy.orm.dynamic.AppenderQuery'>加了lazy就变成了AppenderQuery对象
#因为AppenderQuery对象,就可以这样写，比如说查询该用户31天发表的文章,大大节省了性能
print(user.articles.filter(Article.create_time.day == 31))

#还可以追加数据进去
article = Article(title= 'title100')
user.articles.append(article)
session.commit()