Mongoose MongoDB Objects undefined - node.js

Question

I've run into a strange issue. I've started to use MongoDB and it's most likely me doing something terrible wrong.
I have the following models setup:
var cartSchema = mongoose.Schema({
owner: { type: Schema.Types.ObjectId, ref: 'users' },
productline: [{ type: Schema.Types.ObjectId, ref: 'Productline' }]
});
var productlineSchema = mongoose.Schema({
cart: { type: Schema.Types.ObjectId, ref: 'Cart' },
product: { type: Schema.Types.ObjectId, ref: 'products' },
quantity: Number
});
Initially the Cart is setup with an empty array when the User registers, but then we add Productline objects to it later (which works since I can see data in there in my GUI.).
I'm trying to read the name value which is intended to be reached from cart -> productline -> product
for (var i=0; i < cartModel.productline.length; i++) {
console.log(cartModel.productline[i].product.name);
}
But getting TypeError: Cannot read property 'name' of undefined on exactly that line, which means that product == "undefined".
However, when I'm checking in my MongoDB with MongoDB Compass, then I can see that there is infact a connection between them, and the id's looks accurate as far as I can tell, so it should be able to read it.
So either I'm trying to reach the values in the wrong way, cartModel.productline[0].product.name.
Or my code doesn't realize that the object has been updated, which is strange since I even made sure to use Cart.findOne(query, function(err, cartModel) { ... } to be sure I get a fresh one from the database.
Anyone has any ideas? I'll be glad to post some more code if needed, I just tried to find the most relevant parts above, but I might be missing something somewhere else...

Answer 1

I actually managed to solve this by myself. For people who are having problems with nested objects, I recommend you looking into the mongoose-deep-populate plugin: https://github.com/buunguyen/mongoose-deep-populate
It helped me out a lot, and my final query ended up like this:
Cart.findOne({_id: cart}).deepPopulate('productline.product').exec(function (err, docs) {
// docs is JSON data which can be used to reach the nested data.
});

Answer 2

I have also encountered similar error recently
Demo
When I use: .populate('productline')
Cart.findOne({_id: cart}).populate('productline').exec(function(err, docs) {
/* docs.productline items represent the correct model but their
fields are undefined except the `_id` */
})
Solution
When I use: .populate({path: 'productline'})
Cart.findOne({_id: cart}).populate({path: 'productline'}).exec(function(err, docs) {
/* docs.productline items represent the correct model with all properties
fetched with the correct values */
})
Other solution
This example from http://mongoosejs.com/docs/populate.html helped me
Story
.find(...)
.populate({
path: 'fans',
match: { age: { $gte: 21 }},
select: 'name -_id', /* populates only these fields in the `fans` collection */
options: { limit: 5 }
})
.exec()