Add option to $currentDate to update field only if a update of other fields really occured

Hello,

I don't know if it's possible because I don't know the mechanics inside the update process well enough, but I thought this would be interesting to extend the $currentDate operator to update the date if and only if (optionnaly) at least 1 other fields in document was updated.

As an example, imagine I have a document :

{"_id": 1, "firstname":"Henri", "du":ISODate("2017-04-12T19:10:28.000Z")}

And this update request :

db.test.update({"firstname": "Henri"},

	{

	 	"$set":{"firstname": "Henri"},

		"$currentDate":{"du": true} 

	},

	{

		"upsert": true

	}

);

Appart from the $currentDate, this update does not actually update anything in the document. And I thought this would be useful to have $currentDate update the "du" fields only if firstname had changed.

Maybe there is a design pattern to do that another way, but in my application, I have to do a lot of bulk updates and sometimes, with data that has already been written and thus, updates doing nothing...

Thanks for any input on the matter.