SQLAlchemy join on composite key, can't make it work... :(

sqlalchemy_join.py

# coding=utf-8
from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db'
# app.config['SQLALCHEMY_BINDS'] = {
#     'users':        'mysqldb://localhost/users',
#     'appmeta':      'sqlite:////path/to/appmeta.db'
# }
db = SQLAlchemy(app)


class Asset(db.Model):
    __tablename__ = 'asset'
    user = db.Column('usuario', db.Integer, primary_key=True)
    profile = db.Column('perfil', db.Integer, primary_key=True)
    name = db.Column('nome', db.Unicode(255))

    def __str__(self):
        return u"Asset({}, {}, {})".format(self.user, self.profile, self.name).encode('utf-8')


class Zabumba(db.Model):
    __tablename__ = 'zabumba'
    
    db.ForeignKeyConstraint(
        ['asset.user', 'asset.profile'],
        ['zabumba.user', 'zabumba.profile']
    )

    user = db.Column('usuario', db.Integer, primary_key=True)
    profile = db.Column('perfil', db.Integer, primary_key=True)
    count = db.Column('qtdade', db.Integer)

    def __str__(self):
        return u"Zabumba({}, {}, {})".format(self.user, self.profile, self.count).encode('utf-8')

   
db.drop_all()
db.create_all()

db.session.add(Asset(user=1, profile=1, name=u"Pafúncio"))
db.session.add(Asset(user=1, profile=2, name=u"Skavurska"))
db.session.add(Asset(user=2, profile=1, name=u"Ermengarda"))

db.session.add(Zabumba(user=1, profile=1, count=10))
db.session.add(Zabumba(user=1, profile=2, count=11))
db.session.add(Zabumba(user=2, profile=1, count=12))

db.session.commit()

print "ALL:"
for asset, zabumba in db.session.query(Asset, Zabumba).all():
    print "{:25}\t<---->\t{:25}".format(asset, zabumba)


print "Attempt 1:"
for asset, zabumba in db.session.query(Zabumba).join(Asset).all():
    print "{:25}\t<---->\t{:25}".format(asset, zabumba)

print "Attempt 2:"
for asset, zabumba in db.session.query(Asset).join(Zabumba).all():
    print "{:25}\t<---->\t{:25}".format(asset, zabumba)