6. 关系定义
1. 一对多关系¶
对于一个普通的博客应用来说,用户和文章显然是一个一对多的关系,一篇文章属于一个用户,一个用户可以写很多篇文章,那么他们之间的关系可以这样定义:
class User(Base): __tablename__ = 'users' id = Column(Integer, primary_key=True) username = Column(String(64), nullable=False, index=True) password = Column(String(64), nullable=False) email = Column(String(64), nullable=False, index=True) articles = relationship('Article') def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.username) class Article(Base): __tablename__ = 'articles' id = Column(Integer, primary_key=True) title = Column(String(255), nullable=False, index=True) content = Column(Text) user_id = Column(Integer, ForeignKey('users.id')) author = relationship('User') def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.title)
SQLAlchemy 提供了 backref 让我们可以只需要定义一个关系:
articles = relationship('Article', backref='author')
2. 一对一关系¶
在 User 中我们只定义了几个必须的字段, 但通常用户还有很多其他信息,但这些信息可能不是必须填写的,我们可以把它们放到另一张 UserInfo 表中,这样User 和 UserInfo 就形成了一对一的关系。你可能会奇怪一对一关系为什么不在一对多关系前面?那是因为一对一关系是基于一对多定义的:
class User(Base): __tablename__ = 'users' id = Column(Integer, primary_key=True) username = Column(String(64), nullable=False, index=True) password = Column(String(64), nullable=False) email = Column(String(64), nullable=False, index=True) articles = relationship('Article', backref='author') userinfo = relationship('UserInfo', backref='user', uselist=False) def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.username) class UserInfo(Base): __tablename__ = 'userinfos' id = Column(Integer, primary_key=True) name = Column(String(64)) qq = Column(String(11)) phone = Column(String(11)) link = Column(String(64)) user_id = Column(Integer, ForeignKey('users.id'))
3. 多对多关系¶
一遍博客通常有一个分类,好几个标签。标签与博客之间就是一个多对多的关系。多对多关系不能直接定义,需要分解成俩个一对多的关系,为此,需要一张额外的表来协助完成:
article_tag = Table( 'article_tag', Base.metadata, Column('article_id', Integer, ForeignKey('articles.id')), Column('tag_id', Integer, ForeignKey('tags.id')) ) class Tag(Base): __tablename__ = 'tags' id = Column(Integer, primary_key=True) name = Column(String(64), nullable=False, index=True) def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.name)
4. 映射到数据¶
表已经描述好了,在文件末尾使用下面的命令在我们连接的数据库中创建对应的表:
if __name__ == '__main__': Base.metadata.create_all(engine)
完整代码
# coding: utf-8 import random from faker import Factory from sqlalchemy import create_engine, Table from sqlalchemy.ext.declarative import declarative_base from sqlalchemy import ForeignKey from sqlalchemy import Column, String, Integer, Text from sqlalchemy.orm import sessionmaker, relationship engine = create_engine("mysql+pymysql://root:root@127.0.0.1/blog", encoding='utf-8', echo=False) Base = declarative_base() class User(Base): __tablename__ = 'users' id = Column(Integer, primary_key=True) username = Column(String(64), nullable=False, index=True) password = Column(String(64), nullable=False) email = Column(String(64), nullable=False, index=True) articles = relationship('Article', backref='author') userinfo = relationship('UserInfo', backref='user', uselist=False) def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.username) class UserInfo(Base): __tablename__ = 'userinfos' id = Column(Integer, primary_key=True) name = Column(String(64)) qq = Column(String(11)) phone = Column(String(11)) link = Column(String(64)) user_id = Column(Integer, ForeignKey('users.id')) class Article(Base): __tablename__ = 'articles' id = Column(Integer, primary_key=True) title = Column(String(255), nullable=False, index=True) content = Column(Text) user_id = Column(Integer, ForeignKey('users.id')) cate_id = Column(Integer, ForeignKey('categories.id')) tags = relationship('Tag', secondary='article_tag', backref='articles') def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.title) class Category(Base): __tablename__ = 'categories' id = Column(Integer, primary_key=True) name = Column(String(64), nullable=False, index=True) articles = relationship('Article', backref='category') def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.name) article_tag = Table( 'article_tag', Base.metadata, Column('article_id', Integer, ForeignKey('articles.id')), Column('tag_id', Integer, ForeignKey('tags.id')) ) class Tag(Base): __tablename__ = 'tags' id = Column(Integer, primary_key=True) name = Column(String(64), nullable=False, index=True) def __repr__(self): return '%s(%r)' % (self.__class__.__name__, self.name) if __name__ == '__main__': Base.metadata.create_all(engine) faker = Factory.create() Session = sessionmaker(bind=engine) session = Session() faker_users = [User( username=faker.name(), password=faker.word(), email=faker.email(), ) for i in range(10)] session.add_all(faker_users) faker_categories = [Category(name=faker.word()) for i in range(5)] session.add_all(faker_categories) faker_tags = [Tag(name=faker.word()) for i in range(20)] session.add_all(faker_tags) for i in range(100): article = Article( title=faker.sentence(), content=' '.join(faker.sentences(nb=random.randint(10, 20))), author=random.choice(faker_users), category=random.choice(faker_categories) ) for tag in random.sample(faker_tags, random.randint(2, 5)): article.tags.append(tag) session.add(article) session.commit()
检查mysql
mysql> show tables; +----------------+ | Tables_in_blog | +----------------+ | article_tag | | articles | | categories | | tags | | userinfos | | users | +----------------+ 6 rows in set (0.00 sec)