expressjs + mongoose function save does not work

Here is my code :

UserManager.prototype.Insert = function (acc, pw, key, type) {
db.connect();
db.removeModel('tb_user');

var um = this.CreateModel();

um.find({ account: acc }).then(function (user) {
    var new_user = new um({
        account: acc, password: pw, api_key: key, admin: type
    });
    return new_user.save();
}).then(function (rs) {
    console.log(rs);
});

}

i don't know why i cannot insert new record to database but i replace INSERT to UPDATE, this code working fine Please help me fix it Thanks all!

via Zard