How to three-way many-to-many relationship in flask-sqlalchemy

Question

What's the proper way to design a three-way many-to-many in flask-sqlalchemy?

Assume I have users, teams and roles. Users are assigned to teams. When assigned to a team, the user is also assigned a role within that team.

from myapp import db

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(128), unique=True)

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return "<User(%s)>" % self.name

class Team(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(128), unique=True)

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return "<Team(%s)>" % self.name

class Role(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(128), unique=True)

    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return "<Role(%s)>" % self.name

Several approaches to design a proxy table for this have failed for me. I think I am misunderstanding the docs and thus don't want to confuse you with all the failed approaches I've taken, so far.

I'd like to leave it with the question: What's the proper way to design a three-way many-to-many relationship in flask-sqlalchemy.

Answer 1

dudes. There is official way to solve the problem with two many-many by using a helper table.

Helper_table = db.Table('Helper_table',
                     db.Column('id_a', db.Integer,
                               db.ForeignKey('a.id')),
                     db.Column('id_b', db.Integer,
                               db.ForeignKey('b.id'))
                )

class A(db.Model):  # A ORM
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)

    bs = db.relationship(
        'B', secondary=Helper_table, lazy='dynamic')

    def __repr__(self):
        return '<A {}>'.format(self.username)

class B(db.Model):  # B ORM
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)


    as = db.relationship(
        'A', secondary=Helper_table, lazy='dynamic')

    def __repr__(self):
        return '<B {}>'.format(self.username)

But it can only solve the two many-many problem !!! It will automatically handle the relationship.

a1 = A(username="a1_"+str(uuid.uuid4()))
b1 = B(username="b1_"+str(uuid.uuid4()))

a1.bs.append(b1)
db.session.add(a1)
db.session.commit()

As the code above shows that it will auto add b1 into database as well as b1.as.

But when I tried this method in 3 many-many-many situation, everything f**k up.

Here are the questions I asked: multi-helper table for many-many-many-... relationship in flask-sqlalchemy

Generally, I think it should be a feature of this repo. There should be an elegant way to handle it.

Answer 2

After lots of research and digging it seems I've found the answer, finally. Since I found many bits and pieces of other people having a hard time solving this and couldn't find a complete and clear answer, I figured I could post it here for future travellers.

If you've hit this question, it might be possible that you aren't really looking for a three-way many-to-many. I thought I was, but I wasn't.

Recap: I have users, teams and roles. If a user joins a team, he is also assigned a role within that team.

I went back to the scratch-board and drew what I really wanted:

+---------+---------+---------+
| user_id | team_id | role_id |
+---------+---------+---------+
|       1 |       1 |       1 |
+---------+---------+---------+

Then it started to become clear to me, that I wasn't really looking for a three-way many-to-many, but rather for a three-way one-to-many departing from a forth model.

class Membership(db.Model):
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), primary_key=True)
    team_id = db.Column(db.Integer, db.ForeignKey('team.id'), primary_key=True)
    role_id = db.Column(db.Integer, db.ForeignKey('role.id'), primary_key=True)

    db.UniqueConstraint('user_id', 'team_id', 'role_id')
    db.relationship('User', uselist=False, backref='memberships', lazy='dynamic')
    db.relationship('Team', uselist=False, backref='memberships', lazy='dynamic')
    db.relationship('Role', uselist=False, backref='memberships', lazy='dynamic')

    def __init__(self, user, team, role):
        self.user_id = user.id
        self.team_id = team.id
        self.role_id = role.id

    def __repr__(self):
        return "<Membership(%s)>"

Case of 42: This is exactly the answer I was looking for - I've just asked the wrong question.