Adding data to related table with SQLAlchemy
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)

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

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

MyUser.Friends.Append(MyFriend)

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

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)

>>> 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','user1@admin.com','2')>]
>>> user1.friends[0].added_by
[<User('admin','admin@admin.com','1')>]