ModuleRef.get(Symbol) don't work under some cases

Bug Report

Current behavior

The order of console logs are unexpected 😕 ModuleRef returns an instance of the service without call its constructor.

Input Code

import { Inject, Injectable, Module, Optional } from '@nestjs/common';
import { ModuleRef } from '@nestjs/core';
import { AppController } from './app.controller';
import { AppService } from './app.service';

@Injectable()
class SpecialService {
  constructor(public readonly service: AppService) {
    console.log(`Hey! SpecialService here, my constructor has been called and the AppService is inyected correctly: `, service);
  }
}

const SpecialSymbol = Symbol('Special Symbol');

// If you uncomment any of two comments below (or both), all works as expected
@Module({
  controllers: [AppController],
  providers: [AppService, /* SpecialService, */ { provide: SpecialSymbol, useClass: SpecialService }],
})
export class AppModule {
  constructor(
    @Optional() /* @Inject(SpecialSymbol) */ specialService: SpecialService,
    private moduleRef: ModuleRef,
  ) {
    // It works as expected...
    // console.log(`from ModuleRef`, this.moduleRef.get(SpecialService))
    console.log(`from ModuleRef`, this.moduleRef.get(SpecialSymbol))
    console.log(`from DI`, specialService)
  }
}

Repro: https://github.com/tonivj5/nestjs-bug-module-ref-symbol

• clone
• npm run start
• Play with the comments

Expected behavior

If you uncomment /* SpecialService, */ or /* @Inject(SpecialSymbol) */ it works as expected.

Possible Solution

IDK

Environment

Nest version: 8.0.6

 
For Tooling issues:
- Node version: latest  
- Platform: Linux 

Others:

[jmcdo29]

Okay, I can see the moduleRef.get getting the service before the constructor is called with the current repo. What’s the use case where you’re using the ModuleRef to get the provider in the constructor instead of injecting the provider via the symbol?

What I can tell is happening is that the AppModule class is being instantiated before the SpecialService class, and the ModuleRef can see that there’s this SpecialSymbol token and grabs it, but it doesn’t have a value associated to it yet, it’s just an empty object.

The reason it works properly when you uncomment SpecialService or @Inject(SpecialSymbol) is because that optional dependency now actually exists in the container and Nest waits for it, but when you have @Optional() if the provider doesn’t exist at all, it’ll just be undefined as you’ve seen.

[tonivj5]

Thank you @jmcdo29 & @kamilmysliwiec! I understand that it doesn’t work “by design”, it makes sense.

Just to add my 2 cents here, you should never use moduleRef within the constructor, it’s not designed to be used there.

Yeah, it should be used in a later hook (as onModuleInit)

Thank you guys! 😃