I have a Person entity with its own Repository class, which I want to test. This repository class injects the Mongoose model as suggested in the docs of NestJS, like so:

    @InjectModel(Person.name)
    private model: Model<PersonModel>

And the code I'm trying to test is a query similar to const res = await this.model.find().lean();

My issue, though, is when it es to testing the lean() query, as it's a chained function to find() methods. I was able to get as far as I could, but when it es to mocking it I am having some type conflicts:

const modelMockObject = {
  find: jest.fn(),
  findOne: jest.fn(),
  findOneAndUpdate: jest.fn(),
  updateOne: jest.fn(),
};

// ...

  let MockPersonModel: Model<PersonModel>;

  beforeEach(async () => {
    const mockModule: TestingModule = await Test.createTestingModule({
      providers: [
        ...,
        {
          provide: getModelToken(Person.name),
          useValue: modelMockObject,
        },
      ],
    })pile();

    MockPersonModel = mockModule.get<Model<PersonModel>>(
      Person.name,
    );
  }); 

// ...
// Inside a describe/it test...

      const personModel = new MockPersonModel({
        name: 'etc'
      });

      jest.spyOn(MockPersonModel, 'findOne').mockReturnValueOnce({
        lean: () => ({ exec: async () => personModel }),
      });

The error the linter informs on personModel (second-last line) is the following:

Type 'Promise<PersonModel>' is not assignable to type 'Promise<P>'.
  Type 'PersonModel' is not assignable to type 'P'.
    'P' could be instantiated with an arbitrary type which could be unrelated to 'PersonModel'.ts(2322)
index.d.ts(2100, 5): The expected type es from the return type of this signature.

Thanks a lot for your help!

I have a Person entity with its own Repository class, which I want to test. This repository class injects the Mongoose model as suggested in the docs of NestJS, like so:

    @InjectModel(Person.name)
    private model: Model<PersonModel>

And the code I'm trying to test is a query similar to const res = await this.model.find().lean();

My issue, though, is when it es to testing the lean() query, as it's a chained function to find() methods. I was able to get as far as I could, but when it es to mocking it I am having some type conflicts:

const modelMockObject = {
  find: jest.fn(),
  findOne: jest.fn(),
  findOneAndUpdate: jest.fn(),
  updateOne: jest.fn(),
};

// ...

  let MockPersonModel: Model<PersonModel>;

  beforeEach(async () => {
    const mockModule: TestingModule = await Test.createTestingModule({
      providers: [
        ...,
        {
          provide: getModelToken(Person.name),
          useValue: modelMockObject,
        },
      ],
    }).pile();

    MockPersonModel = mockModule.get<Model<PersonModel>>(
      Person.name,
    );
  }); 

// ...
// Inside a describe/it test...

      const personModel = new MockPersonModel({
        name: 'etc'
      });

      jest.spyOn(MockPersonModel, 'findOne').mockReturnValueOnce({
        lean: () => ({ exec: async () => personModel }),
      });

The error the linter informs on personModel (second-last line) is the following:

Type 'Promise<PersonModel>' is not assignable to type 'Promise<P>'.
  Type 'PersonModel' is not assignable to type 'P'.
    'P' could be instantiated with an arbitrary type which could be unrelated to 'PersonModel'.ts(2322)
index.d.ts(2100, 5): The expected type es from the return type of this signature.

Thanks a lot for your help!

• javascript
• typescript
• mongoose
• jestjs
• nestjs

1 Answer 1

Maybe too late for you, but for the next one who search to solve this problem like me too.

You can simply use mockImplementation: eg:

MockPersonModel.findOne.mockImplementationOnce(() => ({
    lean: jest.fn().mockReturnValue(personModel),
}));

You can check here for more detail about this implementation:

my simple test