Multikind Object validation in Nestjs DTO

Question

I have code something like below. But I found out that @ValidateNested doesn't support when we do "TwoWheel | FourWheel" like this. Also I have no way of determining if incoming payload vehicle type is TwoWheel or FourWheel. I need to validate that as well. How can I do so ?

Please note that these are just dummy classes I have created. In reality, I have 3 classes and each have many fields. Only some of them are common. Most of them are not.

ChatGPT suggested to create custom validator. But I was hoping nestjs might have something inbuilt that I can use. Please suggest better approach.

Thanks in advance.

class TwoWheel{
    @IsNotEmpty()
    @IsNumber()
    speed: number;

    @IsNotEmpty()
    @IsString()
    color: string
}


class FourWheel{
    @IsNotEmpty()
    @IsNumber()
    speed: number;

    @IsNotEmpty()
    @IsString()
    color: string

    @IsNotEmpty()
    @IsNumber()
    numberOfSeats: number;
}

class Vehicle{
    
    @ValidateNested()
    VehicleType : TwoWheel | FourWheel
}

Another thing I tried was something like

    @IsEnum([TwoWheel, FourWheel],{
        message : "Invalid Vehicle Type"
    })
    VehicleType : TwoWheel | FourWheel

But above will always return error message. At least getting @ValidateNested would also be win for me. I need to validate nested field while doing "VehicleType : TwoWheel | FourWheel" Don't want too complex solution. Just want to find out if there is something in nestjs that I missed.

Answer 1

You can use the @Type annotation from class-transformer to manually check the type of the input based on a specific parameter. For example

export class VehicleA {
  @IsNotEmpty()
  doors: number;
}

export class VehicleB {
  @IsNotEmpty()
  doors: number;

  @IsNotEmpty()
  seats: number;
}

export class MyDTO {

  @IsNotEmpty({
    message: "Vehicle should not be empty"
  })
  @ValidateNested({ each: true })
  @Type((options) => {
    if (options?.object && "seats" in options.object.vehicle) {
      return VehicleB;
    }

    return VehicleA;
  })
  vehicle: VehicleA | VehicleB;

}

In this scenario you can use the fact that VehicleB has a property seats and then return the appropriate type, almost like a typeguard.

Edit: Adding here a better answer using the @Type discriminator: https://github.com/typestack/class-transformer?tab=readme-ov-file#providing-more-than-one-type-option