【Flask】ORM 关系一对一

### 一对一的关系：
在sqlalchemy中，如果想要将两个模型映射成一对一的关系，那么应该在父模型中，指定引用的时候，要传递一个`uselist=False`这个参数进去。就是告诉父模型，以后引用这个从模型的时候，不再是一个列表了，而是一个对象了。示例代码如下：

 # 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)

 # UserExtent
 class UserExtend(Base):
     __tablename__ = 'userextend'
     id = Column(Integer, primary_key=True, autoincrement=True)
     school = Column(String(50))
     uid = Column(Integer, ForeignKey("user.id"))

     user = relationship("User", backref=backref("extend", uselist=False))

 # 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()

 # 一对多
 user = User(username='saber')
 # article1 = Article(title='title1', conent='content1')
 # article2 = Article(title='title2', conent='content2')
 # user.articles.append(article1)
 # user.articles.append(article2)
 #
 # session.add(user)
 # session.commit()
 print '-' * 20

 # 一对一, 多对一
 extend1 = UserExtend(school='tinghua')
 user.extend = extend1
 session.add(user)
 session.commit()