Why is execPopulate method required in mongoose for populating an existing document?

Question

I know the syntax of it and how it works, but I cannot understand the internal workings, why does a method chaining require another method at one time, but doesn't some other time?

This code works fine

const cart = await Carts.findById(cartId).populate('product');

But this code does not

let cart = await Carts.findById(cartId);
cart = await cart.populate('product');

And to make it work, we use the execPopulate method which works like this.

let cart = await Carts.findById(cartId);
cart = await cart.populate('product').execPopulate();

Now, as far as I have read method chaining in javascript, the code should run fine without the execPopulate method too. But I cannot seem to understand why populate does not work on existing mongoose objects.

Answer 1

Just a note for anyone reading this that execPopulate() has now been removed: https://mongoosejs.com/docs/migrating_to_6.html#removed-execpopulate

Answer 2

Carts.findById(cartId); returns query Object.

When you use await Carts.findById(cartId); it returns the document as it will resolve the promise and fetch the result.

The await operator is used to wait for a Promise.

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let cart = await Carts.findById(cartId); // cart document fetched by query
cart = await cart.populate('product'); // you can't run populate method on document

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Valid case

const cartQuery = Carts.findById(cartId);
const cart = await cartQuery.populate('product');

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.execPopulate is method on document, while .populate works on query object.