nestjs mock imported module

Question

I'm trying to test my AuthModule controller HTTP layer with supertest as described in the official Nestjs documentation. This module uses an EmailService imported from an EmailModule.

I know you can override providers as follows:

const moduleFixture: TestingModule = await Test.createTestingModule({
      imports: [
        AuthModule,
      ],
    })
      .overrideProvider(EmailService)
      .useValue(buildMock(EmailService))

but this doesn't work, I'm assuming because the EmailModule is imported in the AuthModule. A workaround is to .overrideProvider(...).useValue(...) every provider in the EmailModule, but this non sense as I'd then have to also mock modules imported by the EmailModule.

When I'm e2e testing the AuthModule, I honestly don't care about how the EmailModule works. All I should need to mock is the EmailService and make sure my auth module interacts with that service properly.

The Test.createTestingModule({}) doesn't have a .overrideModule() method, unfortunately.

I tried mocking the EmailModule with jest:

jest.mock('../../src/email/email.module.ts', () => {
  @Module({})
  class EmailModule {

  }

  return EmailModule;
});

but I get this error:

Error: Nest cannot create the module instance. Often, this is because of a circular dependency between modules. Use forwardRef() to avoid it.

(Read more: https://docs.nestjs.com/fundamentals/circular-dependency)
Scope [RootTestModule -> AppModule]

Does anyone know how this can be achieved ?

Answer 1

The way I solved this is by creating a TestModule that is global:

    @Global()
    @Module({
      providers: [
        {
          provide: EmailService,
          useValue: {} // mock
        },
      ],
    })
    class TestModule {}

Import the TestModule at the beginning of your testing module:

    const module = await Test.createTestingModule({
      imports: [
        TestModule,
        AuthModule,
        // ...
      ]
    }).compile();

That way, the EmailService is provided globally and you don't need to import the EmailModule anymore.

Global modules are explained here.