Node.js - Create Relationships with Mongoose from different file - node.js

Question

I have two Schemas Products and Users in different files. Products are belong to User and User have many Product
The Problem is, I have try to use Populate but somehow it return not what I expected.
here is my Schema for Product on models products.js
var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var users = require('../models/users');
var Product = mongoose.Schema({
user_id : {type: String, required: true },
category_id : {type: String, default: null},
title : {type: String, required: true },
content : {type: String, default : "" },
pictureUrls : Array,
counter : {type : Number, default : 0},,
lowercaseTitle : {type: String, default : null },
_user : { type: Schema.Types.ObjectId, ref: users.User }
});
Product.set('toObject', {
getters: true,
virtuals: true
});
module.exports = mongoose.model('Product', Product)
and this is my Schema for User on models users.js
var mongoose = require('mongoose');
var User = mongoose.Schema({
firstName : {type: String, default: "" },
lastName : {type: String, default: "" },
username : {type: String, required: true },
email : {type: String, required: true },
password : {type: String, default: "" },
bio : {type: String, default: "" },
website : {type: String, default: "" },
phone : {type: String, default: "" },
gender : {type: String, default: "" },
birthDate : {type: Date, default: null },
avatarUrl : {type: String, default: "" },
verified : {type : Boolean, default : false}
});
User.set('toObject', {
getters: true,
virtuals: true
});
module.exports = mongoose.model('User', User);
currently I am using method find by calling each Models
User.findOne({"sessionToken" : bearerHeader}, function (err, user){
Product.find({"user_id" : user._id}, function (err, products){
console.log(products);
});
});
but it takes time and became problem if there related to another models.
I'm calling populte with this
Product.findOne({}).populate('_user').exec(function(err, p){
console.log(p);
});
but attribute _user was not set and undefined
any help?
Thanks