以下问题赞为解决：

# class User(Base):
#     __tablename__ =  'user'
#     #表的结构
#     id = Column(String(20), primary_key=True)
#     username = Column(String(20))
#     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'))