Adding data to related table with SQLAlchemy

Question

I have this to SQLAlchemy(using the Flask SqlAlchemy) objects defined:

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), unique=True)
    password = db.Column(db.String(30))
    email = db.Column(db.String(45), unique=True)
    friends = db.relationship('Friend', backref='user',
                                lazy='dynamic')

    def __init__(self, username, password, email):
        self.username = username
        self.password = password
        self.email = email

    def __repr__(self):
        return "<User('%s','%s','%s')>" % (self.username, self.email, self.id)


class Friend(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    userId = db.Column(db.Integer, db.ForeignKey('user.id'))
    friendId = db.Column(db.Integer)
    created = db.Column(db.DateTime)

    def __init__(self, userId, friendId):
        self.userId = userId
        self.friendId = friendId
        self.created = datetime.datetime.now()

    def __repr__(self):
        return "<Friend(%i,%i)>" % (self.userId, self.friendId)

As I understand to add a friend I should be able to do something like this:

First getting the user:

MyUser = bpdata.User.query.filter_by(id=1).first()

Getting the friend:

MyFriend = bpdata.User.query.filter_by(id=2).first()

Now I would like to do:

MyUser.Friends.Append(MyFriend)

Is this possible or do I just have to add the friend IDs directly into the table Friend?

I think you are creating wrong relationship. One user has many friends and same friend can be friend in many user so you have to use M2M instead of O2M. — NPatel
– NPatel, Commented Mar 5, 2012 at 5:24

StefanE · Accepted Answer · 2012-03-05 16:46:38Z

Figured this one out my self..

What I needed to do was:

MyUser.friends.append(Friend(MyUser.id, MyFriend.id))

and then commit the update.

Update:

Ok I found the proper way of doing what I wanted. First I don't need the Friend table/class at all. Full code:

association_table = db.Table('association',
    db.Column('user_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('friend_id', db.Integer, db.ForeignKey('user.id'))
)
class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(20), unique=True)
    password = db.Column(db.String(30))
    email = db.Column(db.String(45), unique=True)
    friends = db.relationship("User",
                secondary=association_table,
                backref='added_by',
                primaryjoin=id == association_table.c.user_id,
                secondaryjoin=id == association_table.c.friend_id)

With this I can now do following:

>>> user1 = User.query.filter_by(id=1).first()
>>> user1.friends
[]
>>> user2 = User.query.filter_by(id=2).first()
>>> user1.friends.append(user2)
>>> user1.friends
[<User('user1','[email protected]','2')>]
>>> user1.friends[0].added_by
[<User('admin','[email protected]','1')>]

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