Mongoose won't update a document

I'm encountering few problems when dealing with mongoose.

I wrote the following snippet of code:

if (!usr.settings) usr.settings = {};
async.forEach(Object.keys(params), function (item, nextitem){
    usr.settings[item] = params[item];
    nextitem();
}, function (err) {
    if (err) return callback(err);
    usr.save(function(err) {
        if (err) return callback(err);
        return callback();
    });
});

When I first use it, it works just fine, it creates all the items in the document perfectly, but when I use it the 2nd time (lets say I want to update those items) it doesn't change their values nor gives me any error. The data stays just the same.

I tried to debug it.

if (!usr.settings) usr.settings = {};
async.forEach(Object.keys(params), function (item, nextitem){
    usr.settings[item] = params[item];
    nextitem();
}, function (err) {
    if (err) return callback(err);
    usr.save(function(err) {
        console.log(usr); <------------------- [At this point it shows the updated data but for some reason it doesnt save it to the db]
        if (err) return callback(err);
        return callback();
    });
});

Any idea why it could happen? Thanks.