【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：

 1 # coding:utf-8
 2 # Author: liangjun.chen
 3 
 4 
 5 from sqlalchemy import create_engine, Column, Integer, String, Text, ForeignKey
 6 
 7 from sqlalchemy.ext.declarative import declarative_base
 8 from sqlalchemy.orm import sessionmaker, relationship, backref
 9 
10 HOSTNAME = '127.0.0.1'
11 PORT = 3306
12 DATABASE = 'sqlalchemy_first'
13 USERNAME = 'root'
14 PASSWORD = '123456'
15 
16 DB_URI = 'mysql+pymysql://{username}:{password}@{host}:{port}/{dbname}?charset=utf8'.format(
17         username=USERNAME, password=PASSWORD, host=HOSTNAME, port=PORT, dbname=DATABASE
18 )
19 engine = create_engine(DB_URI)
20 Base = declarative_base(engine)
21 Session = sessionmaker(engine)
22 session = Session()
23 
24 
25 # user
26 class User(Base):
27     __tablename__ = 'user'
28     id = Column(Integer, primary_key=True, autoincrement=True)
29     username = Column(String(50), nullable=False)
30 
31     # articles = relationship("Article")
32 
33     def __repr__(self):
34         return "User <username: {}>".format(self.username)
35 
36 
37 # UserExtent
38 class UserExtend(Base):
39     __tablename__ = 'userextend'
40     id = Column(Integer, primary_key=True, autoincrement=True)
41     school = Column(String(50))
42     uid = Column(Integer, ForeignKey("user.id"))
43 
44     user = relationship("User", backref=backref("extend", uselist=False))
45 
46 
47 # article
48 class Article(Base):
49     __tablename__ = 'article'
50     id = Column(Integer, primary_key=True, autoincrement=True)
51     title = Column(String(50), nullable=False)
52     conent = Column(Text, nullable=False)
53     uid = Column(Integer, ForeignKey("user.id", ondelete="RESTRICT"))
54 
55     # relation 正向引用, backref反向引用
56     author = relationship("User", backref='articles')
57 
58     def __repr__(self):
59         return "Article <title: {}>".format(self.title)
60 
61 
62 Base.metadata.drop_all()
63 Base.metadata.create_all()
64 
65 # 一对多
66 user = User(username='saber')
67 # article1 = Article(title='title1', conent='content1')
68 # article2 = Article(title='title2', conent='content2')
69 # user.articles.append(article1)
70 # user.articles.append(article2)
71 #
72 # session.add(user)
73 # session.commit()
74 print '-' * 20
75 
76 # 一对一, 多对一
77 extend1 = UserExtend(school='tinghua')
78 user.extend = extend1
79 session.add(user)
80 session.commit()