【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 = ''

 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