server.dart

register(Request request) {
    var user = new User.fromJson(request.json);

    if (!user.emailIsValid || !user.hashIsValid) {
        return new Response('invalid data', status: Status.ERROR);
    }

    return db.open().then((_) {
        DbCollection users = db.collection('Users');

        return users.findOne({'email': user.email}).then((dbUser) {
            if (dbUser == null) {
                user.strengthenHash();

                return users.insert(user.toJson()).then((_) {
                    db.close();
                    return new Response('user created');
                });
            } else {
                db.close();
                return new Response('email exists', status: Status.ERROR);
            }
        });
    });
}