Inject TypeORM repository into NestJS service for mock data testing

Question

There\'s a longish discussion about how to do this in this issue.

I\'ve experimented with a number of the proposed solutions but I\'m not having much luck.

C

Answer 1

Let's assume we have a very simple service that finds a user entity by id:

export class UserService {
  constructor(@InjectRepository(UserEntity) private userRepository: Repository) {
  }

  async findUser(userId: string): Promise {
    return this.userRepository.findOne(userId);
  }
}

Then you can mock the UserRepository with the following mock factory (add more methods as needed):

// @ts-ignore
export const repositoryMockFactory: () => MockType> = jest.fn(() => ({
  findOne: jest.fn(entity => entity),
  // ...
}));

Using a factory ensures that a new mock is used for every test.

describe('UserService', () => {
  let service: UserService;
  let repositoryMock: MockType>;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        UserService,
        // Provide your mock instead of the actual repository
        { provide: getRepositoryToken(UserEntity), useFactory: repositoryMockFactory },
      ],
    }).compile();
    service = module.get(UserService);
    repositoryMock = module.get(getRepositoryToken(UserEntity));
  });

  it('should find a user', async () => {
    const user = {name: 'Alni', id: '123'};
    // Now you can control the return value of your mock's methods
    repositoryMock.findOne.mockReturnValue(user);
    expect(service.findUser(user.id)).toEqual(user);
    // And make assertions on how often and with what params your mock's methods are called
    expect(repositoryMock.findOne).toHaveBeenCalledWith(user.id);
  });
});

For type safety and comfort you can use the following typing for your mocks (far from perfect, there might be a better solution when jest itself starts using typescript in the upcoming major releases):

export type MockType = {
  [P in keyof T]: jest.Mock<{}>;
};