Mongoose populate () returns an empty array without errors

Question

I am trying to get this work to fill, but I am having problems because I am not getting the expected results and no errors for the job. Just an empty array.

My models look like this. Each their own file

var mongoose = require ('mongoose');

var upgradeSchema = new mongoose.Schema({
  type: {
    type: String,
    default: "Any"
  },
  ability: String,
  ability_desc: String,
  level: Number,
  tag: String
});

mongoose.model('Upgrade', upgradeSchema);

and the other

var mongoose = require( 'mongoose' );
var crypto = require('crypto');
var jwt = require('jsonwebtoken');


var userSchema = new mongoose.Schema({
  email: {
    type: String,
    unique: true,
    required: true
  },
  hero: {
    level: Number,
    name: String,
    type: {
      path: String,
      heroType: String
    },
    upgrades: [{
      type: mongoose.Schema.Types.ObjectId, ref: 'Upgrade'
    }],
    unspent_xp: Number,
    total_xp: Number,
  },
  armyTotal: {
    type: Number,
    default: 0,
    max: 5000
  },
  army:[{
    foc_slot: String,
    unit_name: String,
    unit_cost: Number
  }],
  username: {
    type: String,
    required: true,
    unique: true,
  },
  faction: String,
  name: {
    type: String,
    required: true
  },
  hash: String,
  salt: String,
  roles: {
    type: String,
    default: 'player' }
});

And I'm trying to do it

module.exports.profileRead = function(req, res) {


    User
      .findById(req.payload._id)
      .populate('hero.upgrades')
      .exec(function (err, user) {
        if (err){
          console.log(err);
        } else {
          res.status(200).json(user);
          console.log("success");
        }
      });
    }
};

This is an example user.

{
    "_id" : ObjectId("57b4b56ea03757e12c94826e"),
    "hash" : "76",
    "salt" : "2",
    "hero" : {
        "upgrades" : [ 
            "57b42773f7cac42a21fb03f9"
        ],
        "total_xp" : 0,
        "unspent_xp" : 0,
        "type" : {
            "heroType" : "Psyker",
            "path" : ""
        },
        "name" : "Jon Doe"
    },
    "username" : "michaelzmyers",
    "faction" : "Grey Knights",
    "email" : "[e-mail]",
    "name" : "Michael Myers",
    "roles" : "player",
    "army" : [],
    "armyTotal" : 625,
    "__v" : 3
}

, ObjectId , eample, ObjectId ( "STRINGHERE" ) . . , populate ( populate from hero.upgrades ), . , , . , ( ), . . ? , .

Answer 1

, , :

User
  .findById(req.payload._id)
  .populate({
       path: 'hero.upgrades',
       model: 'Upgrade'
   })
  .exec(function (err, user) {
    if (err){
      console.log(err);
    } else {
      res.status(200).json(user);
      console.log("success");
    }
  });
}

, , .. hero.upgrades populate, Mongoose .