多对多关系

多对多的关系，要通过一个中间表进行关联。
中间表，不能通过class的方式实现，只能通过db.Table的方式实现。
设置关联：article_tag = db.Table('article_tag', db.Column('article_id',db.Integer,db.ForeignKey('article.id'),primary_key=True), db.Column('tag_id',db.Integer,db.ForeignKey('tag.id'),primary_key=True) )
源码实现
db_demo2.py

#多对多关系实例
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
import config
import pymysql
pymysql.install_as_MySQLdb()

app = Flask(__name__)
app.config.from_object(config)
db = SQLAlchemy(app)

article_tag = db.Table('article_tag',
                       db.Column('article_id',db.Integer,db.ForeignKey('article.id'),primary_key=True),
                       db.Column('tag_id',db.Integer,db.ForeignKey('tag.id'),primary_key=True)
    )

class Article(db.Model):
    __tablename__='article'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    title = db.Column(db.String(100), nullable=False)
    tags = db.relationship('Tag',secondary=article_tag,backref=db.backref('articles'))

class Tag(db.Model):
    __tablename__ = 'tag'
    id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    name = db.Column(db.String(100), nullable=False)

db.create_all()

@app.route('/')
def hello_world():
    # article1 = Article(title='aaa')
    # article2 = Article(title='bbb')
    # tag1 = Tag(name='111')
    # tag2 = Tag(name='222')
    #
    # article1.tags.append(tag1)
    # article1.tags.append(tag2)
    # article2.tags.append(tag1)
    # article2.tags.append(tag2)
    #
    # db.session.add(article1)
    # db.session.add(article2)
    # db.session.add(tag1)
    # db.session.add(tag2)
    #
    # db.session.commit()

    article1 = Article.query.filter(Article.title == 'aaa').first()
    tags = article1.tags
    # print(111)
    for tag in tags:
        print(tag.name)
    return 'aaa'

if __name__ == '__main__':
    app.run(debug=True)

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