Typescript: $assertPopulated does not preserve the correct type of the document

[ajwootto]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.5.1

Node.js version

16.13.2

MongoDB server version

5.x

Description

The recently-added $assertPopulated helper currently does not preserve the "document" type of a document instance, and instead just turns it into the underlying interface type:

import { Schema, model, connect, Types } from 'mongoose'

interface NestedChild {
    name: string
    _id: Types.ObjectId
}
const nestedChildSchema: Schema = new Schema({ name: String })

interface Parent {
    nestedChild: Types.ObjectId
    name?: string
}

const ParentModel = model<Parent>('Parent', new Schema({
    nestedChild: { type: Schema.Types.ObjectId, ref: 'NestedChild' },
    name: String
}))

const NestedChildModel = model<NestedChild>('NestedChild', nestedChildSchema)

async function run() {
    await connect('mongodb://localhost:27017/')
    const parent = (new ParentModel({
        nestedChild: new NestedChildModel({ name: 'test' }),
        name: 'Parent'
    })).$assertPopulated<{nestedChild: NestedChild}>('nestedChild')

    // works, assertPopulated makes sure `parent` has a nestedChild field correctly typed
    console.log(parent.nestedChild.name)

   // doesn't work, parent is no longer a Document 
    await parent.save()
}

run()

From what I can tell, changing the definition from this:

$assertPopulated<Paths = {}>(paths: string | string[]): Omit<DocType, keyof Paths> & Paths;

to this:

$assertPopulated<Paths = {}>(paths: string | string[]): Omit<this, keyof Paths> & Paths;

would fix the problem.

Steps to Reproduce

See code snippet

Expected Behavior

I would expect that when working with a document instance, $assertPopulated should preserve the document type so that it's possible to save it etc.

[vkarpov15]

We'll look into this, however, we switched this -> DocType for performance reasons. Using this caused some serious slowdowns in tsc.

[vkarpov15]

I switched this back because I confirmed that it doesn't cause a performance regression anymore. I think the performance issues with this were exacerbated by some of the issues we fixed in #10349.

Fix will be in 6.5.4 👍