Nest.js cant't resolve circular dependencencies - nestjs

Question

I am currently running into this nasty problem, and after hours of searching, I have only found a semi-satisfying solution so far. I am curious if anybody has experienced something similar:
I get the usual error message:
ERROR [ExceptionHandler] Nest can't resolve dependencies of the InventoryElementsService (PrismaService, ?, AuthorizerService). Please make sure that the argument dependency at index [1] is available in the InventoryElementsModule context.
Looking at the constructor of InventoryElements.service.ts, I see that the NotificationsSerivce is not properly injected:
#Injectable()
export class InventoryElementsService {
constructor(
private readonly prisma: PrismaService,
private readonly notificationsService: NotificationsService,
private readonly authorizer: AuthorizerService,
) { }
...
}
Looking at the corresponding InventoryElements.module.ts it seems like I do import it properly:
#Module({
imports: [
forwardRef(() => UsersModule),
forwardRef(() => UserPreferenceModule),
forwardRef(() => NotificationsModule),
forwardRef(() => InventoryAttributesModule),
AuthorizerModule
],
providers: [
InventoryElementsResolver,
InventoryElementsService,
PrismaService,
],
exports: [InventoryElementsService]
})
export class InventoryElementsModule {}
It is a circular dependency and therefore I use forwardRef(() => ...).
The imported notifications.module.ts also seems to be initialised correctly and the notifications.service.ts is also marked with Injectable():
#Module({
imports: [
forwardRef(() => UserPreferenceModule),
forwardRef(() => InventoryElementsModule),
EmailModule
],
providers: [
NotificationsResolver,
NotificationsService,
PrismaService
],
exports: [NotificationsService]
})
export class NotificationsModule {}
Now, interestingly, if I inject NotificationsService inline into the InventoryElements.service.ts-constructor directly, it works:
#Injectable()
export class InventoryElementsService {
constructor(
private readonly prisma: PrismaService,
#Inject(forwardRef(() => NotificationsService)) private readonly notificationsService: NotificationsService,
private readonly authorizer: AuthorizerService,
) { }
This solution does not seem ideal, and I am pretty sure that there is something wrong with my code, but I am really running out of options, where to look for mistakes (and yes, I have already double-checked my imports, and all modules are also imported in app.module.ts 🙈)
Thanks for any help, and stay healthy!

Answer 1

You already mentioned what the fix is: add #Inject(forwardRef(() => OtherService)) to both of the service's constructors. forwardRef is necessary in both the module and the provider because both the modules are circular with each other and both of the services are circular with each other. The point of the forwardRef method is to have a lazy evaluator so that Node can end up creating the class references and have function references to the circular classes, without getting into a unresolvable loop of A needs B needs A