nestjs can't inject dependency in custom decorator

Question

In offical document about authorization , i follow this guide and use casl lib

Let's assume that if i want to validate the request man is the owner of an article, i must get the article data from the database, and compare userId between user and article.userId , so i must inject a repoistory dependency like this

@Injectable()
export class DeleteHolePolicyHandler implements IPolicyHandler {
  @Inject()
  private readonly treeholeDaoService: TreeholeDaoService

  async handle(ability: AppAbility, req: Request) {
    const hole = await this.treeholeDaoService.findById(req.body.id)

    return res
  }
}

but i got an error, it shows me this.treeholeDaoService is undefined.

so what should i do that can make it work?

this is reproduce link

Answer 1

You can't inject dependencies since it's an in-place property declared using new. As stated in the docs :

Since we must instantiate the policy handler in-place using the new keyword, ReadArticlePolicyHandler class cannot use the Dependency Injection. This can be addressed with the ModuleRef#get method (read more here ). Basically, instead of registering functions and instances through the @CheckPolicies() decorator, you must allow passing a Type<IPolicyHandler> . Then, inside your guard, you could retrieve an instance using a type reference: moduleRef.get(YOUR_HANDLER_TYPE) or even dynamically instantiate it using the ModuleRef#create method.