【Flask】ORM关系以及一对多

### ORM关系以及一对多：
mysql级别的外键，还不够ORM，必须拿到一个表的外键，然后通过这个外键再去另外一张表中查找，这样太麻烦了。SQLAlchemy提供了一个`relationship`，这个类可以定义属性，以后在访问相关联的表的时候就直接可以通过属性访问的方式就可以访问得到了。示例代码：

# coding:utf-8
# Author: liangjun.chen


from sqlalchemy import create_engine, Column, Integer, String, Text, ForeignKey

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship, backref

HOSTNAME = '127.0.0.1'
PORT = 3306
DATABASE = 'sqlalchemy_first'
USERNAME = 'root'
PASSWORD = '123456'

DB_URI = 'mysql+pymysql://{username}:{password}@{host}:{port}/{dbname}?charset=utf8'.format(
        username=USERNAME, password=PASSWORD, host=HOSTNAME, port=PORT, dbname=DATABASE
)
engine = create_engine(DB_URI)
Base = declarative_base(engine)
Session = sessionmaker(engine)
session = Session()


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

    # articles = relationship("Article")

    def __repr__(self):
        return "User <username: {}>".format(self.username)


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

    # relation 正向引用, backref反向引用
    author = relationship("User", backref='articles')

    def __repr__(self):
        return "Article <title: {}>".format(self.title)
# Base.metadata.drop_all()
# Base.metadata.create_all()
#
# user = User(username='saber')
# session.add(user)
# session.commit()
#
# article = Article(title='abd', conent='123', uid=1)
# session.add(article)
# session.commit()

article = session.query(Article).first()
uid = article.uid
user = session.query(User).get(uid)
print article
print uid
print user

print "-"*20
article = session.query(Article).first()
print article.author.username

print '-'*20
user = session.query(User).first()
print user.articles