Add dynamic key in mongodb on update

I want to add a key with values or update the values if the key exists. So far i managed to do it with. At the moment I am doing this with 2 calls. I am getting the object and then i am altering it. It would like to be able to do this at once. My concern is that function is getting called twice in a short period of time and the second call would not get the updated object. My Problem is that i am Adding a key to the object that is created by another webserver. Is my approach dangerous? Will the second call wait for a completion of the update or do i have to block the db somehow?

How can I add a key on update that is part of an object? The Object with the data looks like this.

{
key: "20170606000000$BgMKClY@$BgYEA10F@", 
value: {
        "quantity" : "0.3",
        "hourbooking_id" : "$BgECBlECDg@"
        }
}

The schema is the following:

var Hours = new Schema({
    _id: ObjectId,
    hours: {}
});

My code looks like this:

this.findHoursById = function(_id, callback){
    Hours.findOne({
        _id: _id
    }, function(err, chours){
        if(err || chours === null){
            callback({
                success: false,
                error: 'User not found'
            })
        }else{
            callback({
                success: true,
                error: 'none',
                hours: chours.hours
            });
        }
    })
};


this.addHours = function(_id, hours, callback){
    var insertHours = {};
    this.findHoursById(_id, function(result){
        if(result.success){
            insertHours = result.hours
        }
        insertHours[hours.key] = hours.value;
        var CHours =  new Hours({
            _id: _id
        });
        CHours.update({$set: {hours: insertHours}}, {upsert: true}, function(err, num, n){
            if (err){
                console.log(err);
                callback({ success: false, error: 'update hours error' });
                //throw err;
            }else{
                callback({ success: true, error: 'none' });
            }
        });
    });
};

via Silve2611