How to mock a MongoDB query .exec() in Jest?

I would like to mock this async function with Jest in a Typescript Express env.

// src/helpers/user.ts
import { UserModel } from "../models";

export const getUserByUsername = async (username: string) => {
  return UserModel.findOne({ username }).exec()
}

// src/models/index.ts
import { getModelForClass } from "@typegoose/typegoose";

export const UserModel = getModelForClass(User, { schemaOptions: { timestamps: true } });

Here is my the test that fails:

// src/helpers/user.test.ts
describe("getUserByUsername():", () => {
  test('should return a user if username matches', async () => {
    const mockUser: User = await userFactory.build("user");

    const spy = jest.spyOn(UserModel, "findOne");
    spy.mockImplementationOnce(() => Promise.resolve(mockUser))

    const userByUsernameResult = usersHelper.getUserByUsername(mockUser.username);

    await expect(userByUsernameResult).resolves.toEqual(expect.objectContaining({ username: mockUser.username })); 
  });
});

The error message I get:

expect(received).resolves.toEqual() Received promise rejected instead of resolved Rejected to value: [TypeError: models_1.UserModel.findOne(...).exec is not a function]

Could you please help me?

UPDATE: I use in some other tests the MongoDB Memory Server approach that works in this case too (see below). However, I would really like to know how to do it by mocking the .exec() (see above).

// src/helpers/user.test.ts
beforeAll(async () => {
  await mongoose.connect(process.env.MONGO_URL as string, { useNewUrlParser: true, useUnifiedTopology: true } as ConnectOptions, (err) => {
    if(err) {
      console.error(err);
      process.exit(1);
    }
  })
});

afterAll(async () => {
  await mongoose.connection.close();
});

describe("getUserByUsername():", () => {
  test('should return a user if username matches', async () => {
    const mockUser: User = await userFactory.build("user", {}, { setId: false });

    const validUser = new UserModel(mockUser);
    await validUser.save();

    const userByUsernameResult = usersHelper.getUserByUsername(mockUser.username);

    await expect(userByUsernameResult).resolves.toEqual(expect.objectContaining({ username: mockUser.username }));
  });

from How to mock a MongoDB query .exec() in Jest?