Flask SQLAlchemy 数据库多对多

from flask import  Flask
from flask_sqlalchemy import SQLAlchemy
import config
app = Flask(__name__)
app.config.from_object(config)
db = SQLAlchemy(app)
book_tag = db.Table('book_tag',#定义关联表
db.Column('book_id',db.Integer,db.ForeignKey('book.id'),primary_key=True),
db.Column('tag_id',db.Integer,db.ForeignKey('shelfing.id'),primary_key=True)
                    )
class Book(db.Model):
    __tablename__='book'
    id = db.Column(db.Integer,primary_key=True,autoincrement=True)
    name = db.Column(db.String(50),nullable=False)
    tags = db.relationship('Shelfing',secondary=book_tag,backref=db.backref('books'))
class Shelfing(db.Model):
    __tablename__='shelfing'
    id = db.Column(db.Integer,primary_key=True,nullable=False)
    tag = db.Column(db.String(50),nullable=False)
db.create_all()
@app.route('/add')
def add():
    book1 = Book(name='Java开发')
    book2 = Book(name='Python游戏编程快速上手')
    book3 = Book(name='文艺范')

    tag1 = Shelfing(tag='文艺')
    tag2= Shelfing(tag='计算机')
    tag3 = Shelfing(tag='技术')

    book1.tags.append(tag2)
    book1.tags.append(tag3)
    book2.tags.append(tag3)
    book3.tags.append(tag1)

    db.session.add_all([book1,book2,book3,tag1,tag2,tag3])
    db.session.commit()
    return '数据添加成功'
@app.route('/select')
def select():
    book = Book.query.filter(Book.name=='Java开发').first()
    tag = book.tags
    for k in tag:
        print(k.tag)

    tag = Shelfing.query.filter(Shelfing.tag=='技术').first()
    book = tag.books
    for k in book :
        print(k.name)
    return '查询数据成功'
@app.route('/')
def hello_world():
    return 'Hello World!'
if __name__ == '__main__':
    app.run()