TypeScript does not return an error when assigning the result of a `lean` query to a variable of type `InstanceType<Model<MyModel>>`

[nikzanda]

Prerequisites

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

Mongoose version

8.4.4

Node.js version

20.10.0

MongoDB server version

6.0.2

Typescript version (if applicable)

5.4.5

Description

Reproduction link: https://stackblitz.com/edit/stackblitz-starters-9hjjfm?file=src%2Findex.ts

TypeScript should not allow me to assign the result of await UserModel.findOne().lean(); to the variable let user: UserInstance | null = null; in fact, the user.set or user.save instruction results in an error.

  let user: UserInstance | null = null;
  if (condition) {
    user = await UserModel.findOne().lean(); // Here it should report an error
    if (user) {
      user.set({
        name: 'John Doe',
      });
      await user.save();
    }
  }

The code below

results in an error when executed:

Steps to Reproduce

• Declare a variable of type InstanceType<Model<MyModel>>;
• Assign the result of await MyModel.findOne().lean();
• Call .set({ ... }) or .save() function of the variabile.

Expected Behavior

TypeScript should not allow the code to compile

[IslandRhythms]

import {
    Schema,
    connection,
    connect,
    Model,
    model,
    SchemaOptions,
    SchemaTypes,
  } from 'mongoose';

  export interface IUser {
    name: string;
    createdAt: Date;
    updatedAt: Date;
  }

  export interface IUserVirtuals {
    id: string;
  }

  type UserModelType = Model<IUser, {}, {}, IUserVirtuals>;

  export type UserInstance = InstanceType<UserModelType>;

  const options: SchemaOptions<IUser> = {
    timestamps: true,
    optimisticConcurrency: true,
  };

  const userSchema = new Schema<IUser, UserModelType>(
    {
      name: {
        type: SchemaTypes.String,
        required: true,
        trim: true,
      },
    },
    options
  );

  const User = model<IUser, UserModelType>('User', userSchema);

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

    await User.create({
        name: 'John',
      });

      let user: UserInstance | null = null;
      user = await User.findOne().lean();
        if (user) {
          user.name = 'John Doe'
          await user.save();
        }
    console.log(user);
  }

  run();

[nikzanda]

await User.create({
        name: 'John',
      });

      let user: UserInstance | null = null;
      user = await User.findOne().lean();
        if (user) {
          user.name = 'John Doe'
          await user.save();
        }
    console.log(user);

Sorry, but your code does not work, as the documentation states:

_The lean option tells Mongoose to skip hydrating the result documents. This makes queries faster and less memory intensive, but the result documents are plain old JavaScript objects (POJOs), not Mongoose documents._

Therefore, the save method cannot be called and TypeScript does not report any errors:

[vkarpov15]

As a workaround, you can do user = await User.findOne().lean<IUser>();. I'm investigating why this issue is happening.

[nikzanda]

Thank you