_id is nor set neither returned in .save function of Model

userUtil.js

var mongoose = require('mongoose');
var myUserModel = require("./players").playerModel;
var util = require("util");

exports.insertUser = function (pn,di,ti){
	var aUserInstance = new myUserModel();
	aUserInstance.pn = pn;
	aUserInstance.di = di;
	aUserInstance.ti = ti;
	aUserInstance.ls = +new Date();
	console.log(aUserInstance);
	var aP = new Object();
	aUserInstance.save(function(err){
		if (err==null){
			console.log("Saved a User document");
				setTimeout(function() {
				console.log("aUserInstance  :>>> "+ util.inspect(aUserInstance, showHidden=true, depth=5));
			  }, 5000);

		}else {
			console.log("Couldn't save " + err);
		}
	
	});
}