Breaking types of `toObject` on SubDocument or ArraySubdocument

[piyushk96]

Prerequisites

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

Mongoose version

8.3.4

Node.js version

20

MongoDB server version

6.6

Typescript version (if applicable)

5.4.5

Description

Since Mongoose v7, the type of the results obtained by calling toObject on SubDocument or ArraySubdocument is any. Prior to v7 toObject return type was working fine.

Steps to Reproduce

** Code for version >=v7 **

import mongoose, { Types, model } from 'mongoose';
const { Schema } = mongoose;

mongoose.connect('mongodb://localhost:27017/testdb');

interface ISub {
  field1: string;
}
interface IMain {
  f1: string;
  f2: ISub & Types.Subdocument;
  f3: (ISub & Types.ArraySubdocument)[];
}

const subSchema = new Schema({ field1: String }, { _id: false });

const mainSchema = new Schema({
  f1: String,
  f2: { type: subSchema },
  f3: { type: [subSchema] }
})
const MainModel = model<IMain>("Main", mainSchema);

const item = new MainModel({
  f1: "test",
  f2: { field1: "test" },
  f3: [{ field1: "test" }]
})

const obj = item.toObject();

const obj2 = item.f2.toObject()

** Code for version < v7 **

import mongoose, { Types, model, Document } from 'mongoose';
const { Schema } = mongoose;

mongoose.connect('mongodb://localhost:27017/testdb');

interface ISub {
  field1: string;
}
interface IMain {
  f1: string;
  f2: ISub & Types.Subdocument;
  f3: (ISub & Types.ArraySubdocument)[];
}

type IMainDoc = IMain & Document;

const subSchema = new Schema({ field1: String }, { _id: false });

const mainSchema = new Schema({
  f1: String,
  f2: { type: subSchema },
  f3: { type: [subSchema] }
})
const MainModel = model<IMainDoc>("Main", mainSchema);

const item = new MainModel({
  f1: "test",
  f2: { field1: "test" },
  f3: [{ field1: "test" }]
})

const obj = item.toObject();

const obj2 = item.f2.toObject()
const obj3 = item.f3[0].toObject()

Expected Behavior

Both obj2 and obj3 should have proper types

[vkarpov15]

In Mongoose 7, we recommend using HydratedSingleSubdocument and HydratedArraySubdocument rather than ISub & Types.Subdocument. With #14612, the following will no longer return any

  interface IMain {
    f1: string;
    f2: HydratedSingleSubdocument<ISub>;
    f3: HydratedArraySubdocument<ISub>[];
  }