/nest-dependency-injection-issue

Example of NestJS dependency injection not resolving to the correct singleton instance

Primary LanguageTypeScript

Example of NestJS (version 9.X) dependency injection not resolving to the correct singleton instance.

Given the following class diagram and making BService declare the constructor dependencies in a different order than CService (i.e. declaring the AService dependency BEFORE the IDatabaseProvider dependency):

classDiagram

BaseEntityService --> IDatabaseProvider
<<interface>> IDatabaseProvider
AService <-- BService
AService <-- CService
BaseEntityService <|-- AService
BaseEntityService <|-- BService
BaseEntityService <|-- CService

@Injectable()
export class BService extends BaseEntityService {
  constructor(
    public aService: AService,
    @Inject(DATABASE_PROVIDER) public database: IDatabaseProvider,
  ) {
    super(database);
  }
}

@Injectable()
export class CService extends BaseEntityService {
  constructor(
    @Inject(DATABASE_PROVIDER) public database: IDatabaseProvider,
    public aService: AService,
  ) {
    super(database);
  }
}

Then the BService singleton instance is generated before the AService singleton instance, and the AService instance injected to the BService singleton instance is not the AService singleton instance exported in AModule:

16:49:25.157 Instantiating BService
16:49:25.161 Instantiating AService
16:49:25.162 Instantiating CService
16:49:25.162 Is AService instance the AService singleton instance?
    BService: false
    CService: true

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