Mongoose populate with array of objects containing ref

I have a Mongoose schema with an array lists of objects that consist of a reference to another collection and a nested array of numbers:

var Schema, exports, mongoose, schema;

mongoose = require("mongoose");

Schema = mongoose.Schema;

schema = new Schema({
  name: {
    type: String,
    required: true,
    unique: true,
    trim: true
  },
  lists: [
    {
      list: {
        type: Schema.ObjectId,
        require: true,
        ref: "List"
      },
      allocations: [
        {
          type: Number,
          required: true
        }
      ]
    }
  ],
  createdAt: {
    type: Date,
    "default": Date.now
  },
  updatedAt: {
    type: Date
  }
});

exports = module.exports = mongoose.model("Portfolio", schema);

However, I cannot get populate to work as expected without getting a TypeError: Cannot read property 'ref' of undefined. I've tried populate('list') and populate('lists list') but I'm either not calling things correctly or my Schema isn't formed correctly. I don't have this problem if I simply reference the lists by themselves:

lists: [
    {
        type: Schema.ObjectId,
        require: true,
        ref: "List"
    }
  ]

but I want to have the allocations array alongside each list. What do I need to do to get the behavior I want?

I found the answer: populate('lists.list') works. Thanks to this question: https://stackoverflow.com/questions/14594511/mongoose-populate-within-an-object?rq=1

lists: [
   {
      list: {
          type: Schema.ObjectId,
          require: true,
          ref: "List"
      },
      allocations: [
          {
              type: Number,
              required: true
          }
      ]
   }
],

=> Because it's an array of objects, you can do this -: Portfolio.populate('lists.list');

lists: [
    {
        type: Schema.ObjectId,
        require: true,
        ref: "List"
    }
]

=> Because it's an array, you just have to do this -: Portfolio.populate('lists');

Actual answer:

Use this,

populate('lists.list')

Extra:

Here are lists are an array of objects (list). In JS you can access this like that,

console.log(lists.list);

and MongoDB syntax is 99% similar to JS syntax. so....................

if you have nested schema

YourSchema.find()
   .populate({
        path: 'map_data',
        populate: {
            path: 'location' 
        }
})

it's work for me

// Cart schema  
var CartSchema = new mongooseSchema({
    productDetails: [{
        productId: {
            type: mongoose.Schema.ObjectId,
            required: true,
            ref: 'Product'

        },
        productCount: Number,
    }],
    UserId: {
        type: String,
        default: '',
        required: true,
        trim: true,
    },
    shopId: {
        type: String,
        default: '',
        required: true,
        trim: true,
    },
});

// add this .populate('productDetails.productId').

db.Cart.find({
    UserId: userId,
    shopId: shopId
}).populate('productDetails.productId')
.skip(pagination.skip)
.limit(pagination.limit)
.exec(function(error,

CartList) {

    if (error) {
        callback(error, null)
    } else {
        callback(null, CartList)
    }
});

Populate didn't work in my case when nesting to array my Schema was

const chatSchema = mongoose.Schema({
    chats: [
    {
     type: Schema.Types.ObjectId,
     ref: "ChatMsg" },
   ],
  })

How I solved it

Chat.findOne(
 { customer: req.body.customer, agency: req.body.agency },
  (err, chat) => {
   if (err) {
    res.status(422).send("Our fault");
   }
    chat.populate("chats").execPopulate(() => {
    res.send(chat);
   });
  }
);