class User(Base):
    __tablename__ = 'user'

>>>    id = Column(String(20), primary_key=True)
    name = Column(String(20))
    # 一对多:
    books = relationship('Book')

class Book(Base):
    __tablename__ = 'book'

    id = Column(String(20), primary_key=True)
    name = Column(String(20))
    # “多”的一方的book表是通过外键关联到user表的:
    user_id = Column(String(20), ForeignKey('user.id'))