Nestjs unit test: TypeError: this.userModel.findById(...).exec is not a function - unit-testing

Question

using nestjs framework and with a repository class that uses mongoose to do the CRUD operations we have a simple users.repository.ts file like this:
#Injectable()
export class UserRepository {
constructor(#InjectModel(User.name) private userModel: Model<UserDocument>) {}
async create(createUserInput: CreateUserInput) {
const createdUser = new this.userModel(createUserInput);
return await createdUser.save();
}
}
async findById(_id: MongooseSchema.Types.ObjectId) {
return await this.userModel.findById(_id).exec();
}
and it works normally when the server is up.
consider this users.repository.spec file :
import { Test, TestingModule } from '#nestjs/testing';
import { getModelToken } from '#nestjs/mongoose';
import { Model } from 'mongoose';
// User is my class and UserDocument is my typescript type
// ie. export type UserDocument = User & Document; <-- Mongoose Type
import { User, UserDocument } from '../domain/user.model';
import { UserRepository } from './users.repository';
//import graphqlScalars from 'graphql-scalar-types';
describe('UsersRepository', () => {
let mockUserModel: Model<UserDocument>;
let mockRepository: UserRepository;
beforeAll(async () => {
const module: TestingModule = await Test.createTestingModule({
providers: [
{
provide: getModelToken(User.name),
useValue: Model, // <-- Use the Model Class from Mongoose
},
UserRepository,
//graphqlScalars,
],
}).compile();
// Make sure to use the correct Document Type for the 'module.get' func
mockUserModel = module.get<Model<UserDocument>>(getModelToken(User.name));
mockRepository = module.get<UserRepository>(UserRepository);
});
it('should be defined', () => {
expect(mockRepository).toBeDefined();
});
it('should return a user doc', async () => {
// arrange
const user = new User();
const userId = user._id;
const spy = jest
.spyOn(mockUserModel, 'findById') // <- spy
.mockResolvedValue(user as UserDocument); // <- set resolved value
// act
await mockRepository.findById(userId);
// assert
expect(spy).toBeCalled();
});
});
so my question:
for the should return a user doc test i get TypeError: metatype is not a constructor when and i guess
.mockResolvedValue(user as UserDocument);
should be fixed.
Note:graphql is used the query to the API and i have no idea that if the scalars should be provieded or not, if i uncomment the scalar, the expect(mockRepository).toBeDefined(); test would not pass any more
so any idea to fix the test would be apreciated.

Answer 1

to handle a chained .exec we should define it via mockReturnThis():
static findById = jest.fn().mockReturnThis();
I needed the constructor to be called via new so i preferd to define a mock class in this way:
class UserModelMock {
constructor(private data) {}
new = jest.fn().mockResolvedValue(this.data);
save = jest.fn().mockResolvedValue(this.data);
static find = jest.fn().mockResolvedValue(mockUser());
static create = jest.fn().mockResolvedValue(mockUser());
static remove = jest.fn().mockResolvedValueOnce(true);
static exists = jest.fn().mockResolvedValue(false);
static findOne = jest.fn().mockResolvedValue(mockUser());
static findByIdAndUpdate = jest.fn().mockResolvedValue(mockUser());
static findByIdAndDelete = jest.fn().mockReturnThis();
static exec = jest.fn();
static deleteOne = jest.fn().mockResolvedValue(true);
static findById = jest.fn().mockReturnThis();
}