Service can't find dependency because of another services injection

Question

I have a NestJs application where a previously working service throws an error (ReviewService can't find DoctorLoaderService) after I use dependency injection on another two services (PmsConnectionService -> UserService). All of these are from separate Modules.

My nestjs version is 5.1.0. PmsConnectionService is part of a module I newly created. I've had to use forwardRef to connect toModule holding the DoctorLoaderService. Notice in the code below that the PmsConnectionService injects DoctorClinicService, which is injecting the DoctorLoaderService. When I comment out the DoctorClinicService in PmsConnectionService, the error disappears. However I still don't know why the error has occured in the first place.

Services

@Injectable()
export class PmsConnectionService {
    constructor(
        private clinicService: DoctorClinicService,
    ) {}

@Injectable()
export class UserService {
  constructor(
    private readonly connectionService: PmsConnectionService,
  ) { }

@Injectable()
export class DoctorLoaderService {
  constructor(
    @Inject(forwardRef(() => ReviewService))
    private readonly reviewService: ReviewService,
  ) {}

@Injectable()
export class ReviewService {
  constructor(
    private readonly doctorLoader: DoctorLoaderService,
    private readonly reviewManager: ReviewManagerService,
  ) { }

Modules:

@Module({
    imports: [
        forwardRef(() => DoctorModule),
        TypeOrmModule.forFeature([
            ...
          ]),
    ],
    providers: [
        PmsConnectionService,
    ],
    exports: [
        PmsConnectionService,
    ],
})
export class PmsModule {}

@Module({
  imports: [
    PmsModule,
    TypeOrmModule.forFeature([
      User,
      ...
    ]),
  ],
  controllers: [
    UserController,
  ],
  providers: [
    UserService,
  ],
  exports: [UserService],
})
export class UserModule {
  public configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(ActivityMiddleware)
      .exclude({
        path: '*',
        method: RequestMethod.OPTIONS,
      })
      .forRoutes({ path: '*', method: RequestMethod.ALL });
  }
}

@Module({
  imports: [
    UserModule,
    TypeOrmModule.forFeature([
      User,
      DoctorProfile,
      DoctorClinic,
      ...
    ]),
    forwardRef(() => BookingModule),
  ],
  controllers: [
    DoctorProfileController,
  ],
  providers: [
    DoctorLoaderService,
    DoctorClinicService,
  ],
  exports: [
    DoctorClinicService,
    DoctorLoaderService,
  ],
})
export class DoctorModule { }

Module({
  imports: [
    PmsModule,
    UserModule,
    DoctorModule,
    TypeOrmModule.forFeature([
      ...
    ]),
  ],
  controllers: [
   ...
  ],
  providers: [
    ReviewService,
  ],
  exports: [
    ReviewService,
  ],
})
export class BookingModule { }

On start the application throws a following error:

[Nest] 17921   - 9/26/2019, 9:44:15 PM   [NestFactory] Starting Nest application...
[Nest] 17921   - 9/26/2019, 9:44:15 PM   [ExceptionHandler] Nest can't resolve dependencies of the ReviewService (?, ReviewManagerService, NotiIssuerService). Please make sure that the argument at index [0] is available in the current context. +90ms
Error: Nest can't resolve dependencies of the ReviewService (?, ReviewManagerService, NotiIssuerService). Please make sure that the argument at index [0] is available in the current context.
    at Injector.resolveSingleParam (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:115:19)
    at Promise.all.dependencies.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:85:49)
    at Array.map (<anonymous>)
    at Injector.resolveConstructorParams (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:83:58)
    at Injector.loadInstance (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:63:20)
    at Injector.loadInstanceOfComponent (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/injector.js:40:20)
    at Promise.all.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:39:102)
    at Array.map (<anonymous>)
    at InstanceLoader.createInstancesOfComponents (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:39:59)
    at Promise.all.map (/Users/dude/Documents/lekar/unexpected_koala_backend/node_modules/@nestjs/core/injector/instance-loader.js:26:24)

I can't figure out how to fix this though I think the problem lies within the connection between PmsConnectionService and DoctorClinicService.

Answer 1

It looks like you only have half of your forward ref set up. As your ReviewService (part of the BookingModule ) depends on your DoctorLoaderService (part of the DoctorModule ) and your DoctorLoaderService depends on your ReviewService , you need to forwardRef in both the module and service of each affected class. In this case DocotrModule needs a forwardRef(() => BookingModule) and BookingModule needs a forwardRef(() => DoctorModule) as well as ReviewService and DoctorLoadingService having similar @Inject(forwardRef(() => ) setups.

In general it is best to avoid these circular dependencies as they can create headaches that are difficult to debug. Maybe see if there is a change in architecture you can take to make the code simpler/less reliant on other parts.