【Flask】ORM關係以及一對多

### ORM關係以及一對多：
mysql級別的外鍵，還不夠ORM，必須拿到一個表的外鍵，而後經過這個外鍵再去另一張表中查找，這樣太麻煩了。SQLAlchemy提供了一個`relationship`，這個類能夠定義屬性，之後在訪問相關聯的表的時候就直接能夠經過屬性訪問的方式就能夠訪問獲得了。示例代碼：

 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 # article
38 class Article(Base):
39     __tablename__ = 'article'
40     id = Column(Integer, primary_key=True,autoincrement=True)
41     title = Column(String(50), nullable=False)
42     conent = Column(Text, nullable=False)
43     uid = Column(Integer, ForeignKey("user.id", ondelete="RESTRICT"))
44 
45     # relation 正向引用, backref反向引用
46     author = relationship("User", backref='articles')
47 
48     def __repr__(self):
49         return "Article <title: {}>".format(self.title)
50 # Base.metadata.drop_all()
51 # Base.metadata.create_all()
52 #
53 # user = User(username='saber')
54 # session.add(user)
55 # session.commit()
56 #
57 # article = Article(title='abd', conent='123', uid=1)
58 # session.add(article)
59 # session.commit()
60 
61 article = session.query(Article).first()
62 uid = article.uid
63 user = session.query(User).get(uid)
64 print article
65 print uid
66 print user
67 
68 print "-"*20
69 article = session.query(Article).first()
70 print article.author.username
71 
72 print '-'*20
73 user = session.query(User).first()
74 print user.articles