If value of a property is null when updating then that property should not be added to the record

Suppose I have a mongoose schema like this:

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var testSchema = new Schema({
    name: {type: String, required: true},
    nickName: {type: String}
});

var Test = module.exports = mongoose.model('Test', testSchema);

I declare methods for CRUD operation using variable Test. From that one such method is update, which is defined as follows:

module.exports.updateTest = function(updatedValues, callback) {

    console.log(updatedValues); //this output is shown below

    Test.update(
        { "_id": updatedValues.id },
        { "$set" : { "name" : updatedValues.name, "nickName" : updatedValues.nickName } },
        { multi: false },
        callback
    );

};

Now, I use this method inside my node router as follows:

router.put('/:id', function(req, res, next) {

    var id = req.params.id,
    var name = req.body.name,
    var nickName = req.body.nickName

    req.checkBody("name", "Name is required").notEmpty();

    var errors = req.validationErrors();

    if(errors) { ........ }
    else {

        var testToUpdate = new Test({
            _id: id,
            name: name,
            nickName: nickName || undefined
        });

        Test.updateTest(testToUpdate, function(err, result) {
            if(err) { throw(err); }
            else { res.status(200).json({"success": "Test updated successfully"}); }
        });

    }
});

Now if I save a new record in database and then see it in database then it looks like:

{
    "_id" : ObjectId("ns8f9yyuo32hru0fu23oh"), //some automatically generated id 
    "name" : "firstTest",
    "__v" : 0
}

Now if I update the same document without changing anything and then if I take a look at same record in database, then I get:

{
    "_id" : ObjectId("ns8f9yyuo32hru0fu23oh"), //some automatically generated id 
    "name" : "firstTest",
    "__v" : 0,
    "nickName" : null
}

Can you see that nickName is set to null? I don't want it to work like this. I want that if my property is null, then that property should not be included in the record.

If you remember, I have console logged the updatedValues before updating it. (see the second code block in question). So, here is the logged values:

{
    "_id" : ObjectId("ns8f9yyuo32hru0fu23oh"), //some automatically generated id 
    "name" : "firstTest",
    "__v" : 0
}

I don't know why, but nickName is not present in the logged values and then after update I get nickName: null. I think, the problem lies in second Code block. Can you please check it?

via Vishal