flask sqlchemy 多对多的自引用关系定义

多对多的定义可以使用关联表，或者重新定义一个模型，通过模型定义多对多的自引用在flask web开发书里有讲到，这里主要演示用关联表定义的方法。

from flask_sqlalchemy import SQLAlchemy
from flask import Flask

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
db = SQLAlchemy(app)
follows=db.Table('follows',
                 db.Column('follower_id',db.Integer,db.ForeignKey('users.id')),
                 db.Column('followed_id',db.Integer,db.ForeignKey('users.id')))
class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True, index=True)

    followers=db.relationship('User',
                              secondary=follows,
                              primaryjoin=id==follows.c.followed_id,
                              secondaryjoin=id==follows.c.follower_id,
                              backref=db.backref('followed',lazy='dynamic'),
                              lazy='dynamic')
u1,u2=User(username='u1'),User(username='u2')
u1.followed.append(u2)
print "u1 following :",u1.followed[0].username
print "u2 followers :",u2.followers[0].username

表中定义了关注的人，和粉丝两个字段。主要需要设置primaryjoin跟secondaryjoin参数