`string` type assignment fails for `ObjectId` in nested subdocuments

[E1ecti]

Prerequisites

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

Mongoose version

9.7.3

Node.js version

26.4.0

MongoDB server version

Unrelated

Typescript version (if applicable)

6.0.3

Description

Related to #16316.

When using nested interfaces (for example, when using class-validator where subdocuments are defined by separate classes), the error Index signature for type 'string' is missing in type 'N' occurs, or Type 'string' is not assignable to type 'ObjectId ...' in the case of nested arrays.

This issue only affects cases where string is used instead of mongoose.Types.ObjectId.

It can also be solved by adding [x: string]: {} to the validation class, however, this is not a suitable solution.

Steps to Reproduce

import * as mongoose from "mongoose"

interface User {
	dummy: string
}

interface Other {
	nested: {
		to: mongoose.PopulatedDoc<User>
	}
}

interface Nested {
	to: string
}
interface OtherNested {
	nested: Nested
}

const userSchema = new mongoose.Schema({ dummy: String })
const userModel = mongoose.model<User>("user", userSchema)

const otherSchema = new mongoose.Schema({
	nested: {
		to: { type: mongoose.Types.ObjectId, ref: "user" }
	}
})
const otherModel = mongoose.model<Other>("other", otherSchema)

async function main() {
	const user = await userModel.create({ dummy: "hello" })

	const userId = user._id.toString()
	const data: OtherNested = { nested: { to: userId } }

	// Index signature for type 'string' is missing in type 'Nested'.
	const other = await otherModel.create(data)

	console.log("Done", other)

	await mongoose.disconnect()
}

main()

Expected Behavior

Using string should be acceptable as using ObjectId inside subdocuments during creation.

[vkarpov15]

I put in a PR to fix, however you can also work around this by making Nested a type instead of an interface:

type Nested = {
        to: string
} 

interface OtherNested {
        nested: Nested
}

[hasezoey]

FYI this is a issue (from the same user) that was also raised in typegoose's discord with reproduction in typegoose-testing.