I have run into what seems to be a bug but I have a feeling I am not using something correctly since the behavior is a little strange.

Goal:
I am trying to set up Mongo to auto increment IDs for me so that when I store documents into the database they are given a unique, incrementing id based on the collection they are in.

On app startup my program, through the java driver, inserts the following documents:

db.system.js

{
		_id : "incID" ,
		value : function (x) {
			var num = db.incrementers.findOne({_id: x}).value;
			db.incrementers.updateOne({_id: x}, { "$inc": { value: 1}});
			return num;
		}
}

db.incrementers

[
  { _id: "test", value: 1}
]

the function takes in the id of the incrementer, adds one and returns the value so the document can be indexed. This all works fine.

The problem comes when I try to insert a document that uses this function.
in the mongo shell if i insert the document:

{ testID: incID('test') }

the function is called and the result is correctly stored into the document. However, if I insert the same document into mongo using the java driver:

new Document( "testID", new BsonJavaScript("incID('test')"); 

the function call itself is stored and not the result of the function.

While I have a feeling that the intended purpose of the BsonJavaScript object is to store the js code itself (and not run it), how am i supposed to run the code on insert?