Testing mongoose models with NestJS

Question

I'm using the mongoose module from NestJS so I have my schema and an interface, and in my service I use @InjectModel to inject my model. I do not realize how I can mock the model to inject in my service.

My service looks like this:

    @Injectable()
    export class AuthenticationService {

        constructor(@InjectModel('User') private readonly userModel: Model<User>) {}

        async createUser(dto: CreateUserDto): Promise<User> {
            const model = new this.userModel(dto);
            model.activationToken = this.buildActivationToken();
            return await model.save();
          }
    }

and in my service test, I have this:

    const mockMongooseTokens = [
      {
        provide: getModelToken('User'),
        useValue: {},
      },
    ];

    beforeEach(async () => {
        const module: TestingModule = await Test.createTestingModule({
          providers: [
            ...mockMongooseTokens,
            AuthenticationService,
          ],
        }).compile();

        service = module.get<AuthenticationService>(AuthenticationService);
      });

But when I run the test I got this error:

    TypeError: this.userModel is not a constructor

I would also like to get my model to perform unit tests over it, as is shown in this article