I working on NestJS app and trying to validate some field depending on other field.

export class CreateOrderDTO {
  @IsDeliveryCodeValid() // Custom validator
  delivery_code: string;

  @IsAddressRequired() // Custom validator
  @IsNotEmptyObject()
  @ValidateNested()
  @Type(() => CreateOrderAddressDTO)
  address: CreateOrderAddressDTO;
}

In this code, delivery_code is unique id of DeliveryType which has needAddress boolean field. I want to validate address only if this needAddress field is set to true.

What I’m trying to achieve is create some custom decorator which checks if provided delivery_code is code of one of delivery types and if found delivery type has needAddress field set to true, if so we have to validate address, otherwise we can ignore validators.

My ideas:

• It could be done in controller but I really want to check this at CreateOrderDTO level.
• Somehow transform whole DTO before validators run and add field needAddress to it’s body so validators can access it.
• Manually transform DTO at @IsDeliveryCodeValid() decorator and add field needAddress to DTO body. It’s not really elegant way because I dont think i should transform dto in validators, it’s not their purpose and can be confusing.

Normally i would use @ValidateIf() but it doesnt accept async functions, so i can’t use a DB query here.

Thanks for your help!