Accessing the .toObject() of a subdocument inside of a method raises a typing error

[viganll]

Prerequisites

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

Mongoose version

8.3.3

Node.js version

18.17.1

MongoDB server version

5

Typescript version (if applicable)

4.8

Description

I have followed the guide to create types for sub-documents in mongoose. While, the approach works in normal code flow, when i try to access the subdocument properties such as .toObject or .ownerDocument inside a method it raises a typescript error.

Steps to Reproduce

import mongoose, { Types } from "mongoose";

// Subdocument definition
interface Names {
  _id: Types.ObjectId;
  firstName: string;
}

// Document definition
interface User {
  names: Names;
}

// Define property overrides for hydrated documents
type THydratedUserDocument = {
  names?: mongoose.Types.Subdocument<Names>;
};

type UserMethods = {
  getName(): string;
};

type UserModelType = mongoose.Model<User, {}, UserMethods, {}, THydratedUserDocument>;

const userSchema = new mongoose.Schema<User, UserModelType, UserMethods>(
  {
    names: new mongoose.Schema<Names>({ firstName: String }),
  },
  {
    methods: {
      getName() {
        // `this.names` is a subdocument, so we can call `toObject` on it
        // -> However, there is a typing error "Property 'toObject' does not exist on type 'Names'."
        return this.names.toObject();
      },
    },
  }
);
const UserModel = mongoose.model<User, UserModelType>("User", userSchema);

async function run() {
  await mongoose.connect("mongodb://localhost:27017");
  await mongoose.connection.dropDatabase();

  const doc = new UserModel({ names: { _id: "0".repeat(24), firstName: "foo" } });
  doc.names?.ownerDocument(); // Works, `names` is a subdocument!
}

run();

Expected Behavior

I would expect that there is no typing error when i try to access the subdocument props such as .toObject inside of a method.

[vkarpov15]

You also need to set THydratedDocumentType generic on Schema constructor to ensure you get the correct this type in methods and middleware. See the following script for a complete example.

import mongoose, { Types } from "mongoose";

// Subdocument definition
interface Names {
  _id: Types.ObjectId;
  firstName: string;
}

// Document definition
interface User {
  names: Names;
}

// Define property overrides for hydrated documents
type THydratedUserDocument = {
  names?: mongoose.HydratedSingleSubdocument<Names>;
};

type UserMethods = { 
  getName(): Names | undefined;
};

type UserModelType = mongoose.Model<User, {}, UserMethods, {}, THydratedUserDocument>;

const userSchema = new mongoose.Schema<
  User,
  UserModelType,
  UserMethods,
  {},
  {},
  {},
  mongoose.DefaultSchemaOptions,
  User,
  THydratedUserDocument
>(
  {
    names: new mongoose.Schema<Names>({ firstName: String }),
  },
  {
    methods: {
      getName() {
        const str: string | undefined = this.names?.firstName;
        return this.names?.toObject(); // Works, `this.names` is a subdoc
      },
    },
  }
);
const UserModel = mongoose.model<User, UserModelType>("User", userSchema);

async function run() {
  await mongoose.connect("mongodb://localhost:27017");
  await mongoose.connection.dropDatabase();

  const doc = new UserModel({ names: { _id: "0".repeat(24), firstName: "foo" } });
  const str: string | undefined = doc.names?.firstName;
  doc.names?.ownerDocument(); // Works, `names` is a subdocument!
}

run();

[viganll]

@vkarpov15 Thank you for looking into it.

While your example works, I still have two questions:

1. What is the role of HydratedSingleSubdocument?
2. In your example the main doc is not properly typed as now it is typed as THydratedUserDocument . e.g doc.getName() raises an error. Although there might be workarounds, it seems to add a layer of complexity. What would be the recommended way to handle it?

Would this be the right way?

type UserHydratedDocument = mongoose.HydratedDocument<User, UserMethods>;

// Define property overrides for hydrated documents
interface THydratedUserDocument extends Omit<UserHydratedDocument, "names"> {
  names?: mongoose.HydratedSingleSubdocument<Names>;
}

[viganll]

In addition, even after the adjustment you suggested, this refers to the lean User instead of hydrated version inside the required function e.g

  age: {
      type: Number,
      required: function () {

       this.names?.toObject() // typescript raises an error
        return true
      },
 },