【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 = '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)


# 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()