Flask - Many to Many Relationship

background

• User can be a follow and be followed by another user!
• one user can follow many users, user can get many followers

Code - Make Many to Many relationship

• In models.py
• follwers doesn’t need to be a Model class since this is an auxiliary table that has no data other than the foreign keys, I created it without an associated model class.
• if you want to know detail, check this original blog [https://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-part-viii-followers]

followers = db.Table('followers',
    db.Column('follower_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('followed_id', db.Integer, db.ForeignKey('user.id'))
)

class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    password_hash = db.Column(db.String(128))
    posts = db.relationship('Post', backref='author', lazy='dynamic') # post.author ==> user instance
    about_me = db.Column(db.String(140))
    last_seen = db.Column(db.DateTime, default=datetime.utcnow)
    followed = db.relationship('User',
        secondary=followers,
        primaryjoin=(followers.c.follower_id == id),
        secondaryjoin=(followers.c.followed_id == id),
        backref=db.backref('followers', lazy='dynamic'), lazy='dynamic')

• Paduck followed Darly
• Darly has follwers Paduck
• To follow fan.followed.append(star)
• To unfollow fan.followed.remove(star)
• To check followed fan.followed.all()
• To check following star.followers.all()

>>> a = User.query.get(1)
>>> a
<User Paduck>
>>> b = User.query.get(2)
>>> b
<User susan>
>>> a.followed.append(b)
>>> db.session.commit()
>>> a.followed.all()
[<User Darly>, <User susan>]
>>> b.followers.all()
[<User Paduck>]
>>> a.followed.remove(b)
>>> db.session.commit()
>>> a.followed.all()
[<User Darly>]
>>> b.followers.all()
[]
>>>

Code - Make it simple by create model functions

    def follow(self, user):
        if not self.is_following(user):
            self.followed.append(user)

    def unfollow(self, user):
        if self.is_following(user):
            self.followed.remove(user)

    def is_following(self, user):
        return self.followed.filter(
            followers.c.followed_id == user.id).count() > 0

>>> a = User.query.get(1)
>>> b = User.query.get(2)
>>> a.is_following(b)
False
>>> a.follow(b)
>>> db.session.commit()
>>> a.is_following(b)
True
>>> a.unfollow(b)
>>> db.session.commit()
>>> a.is_following(b)
False