How do you implement discriminated union body validation where the DTO shape depends on a type field in NestJS?

Q: How do you implement discriminated union body validation where the DTO shape depends on a type field in NestJS?

export class EmailNotificationDto { @IsEmail() recipient: string; @IsString() subject: string; } export class SmsNotificationDto { @IsPhoneNumber() phoneNumber: string; @MaxLength(160) message: string; } export class SendNotificationDto { @IsEnum(['email', 'sms']) type: 'email' | 'sms'; @ValidateNested() @Type(() => Object, { discriminator: { property: 'type', subTypes: [ { value: EmailNotificationDto, name: 'email' }, { value: SmsNotificationDto, name: 'sms' }, ], }, keepDiscriminatorProperty: false, }) payload: EmailNotificationDto | SmsNotificationDto; } @Post('notifications') send(@Body() dto: SendNotificationDto) { if (dto.type === 'email') return this.notifier.sendEmail(dto.payload as EmailNotificationDto); return this.notifier.sendSms(dto.payload as SmsNotificationDto); } The discriminator.property field must exist in the incoming JSON to select the correct subtype. @ValidateNested() must accompany the discriminator @Type() for class-validator to run on the subtype. Each subtype class has its own class-validator decorators that are validated independently. If the discriminator value does not match any subType, class-transformer returns a plain object. keepDiscriminatorProperty: false removes the type field from the instantiated subtype object.

Difficulty: 6/10

DTO validation, discriminated unions, NestJS pipes

Use @Type() from class-transformer with a discriminator option that maps the type field value to specific subtype classes. Pair with @ValidateNested() so class-validator runs on the instantiated subtype. class-transformer reads the discriminator property and instantiates the correct class before validation runs.

Discriminated union DTO validation

Discriminated union validation rules:

1
The discriminator.property field must exist in the incoming JSON to select the correct subtype.
2
@ValidateNested() must accompany the discriminator @Type() for class-validator to run on the subtype.

How do you implement discriminated union body validation where the DTO shape depends on a type field in NestJS?

Difficulty: 6/10

DTO validation, discriminated unions, NestJS pipes

Use @Type() from class-transformer with a discriminator option that maps the type field value to specific subtype classes. Pair with @ValidateNested() so class-validator runs on the instantiated subtype. class-transformer reads the discriminator property and instantiates the correct class before validation runs.

Discriminated union DTO validation

Discriminated union validation rules:

1
The discriminator.property field must exist in the incoming JSON to select the correct subtype.
2
@ValidateNested() must accompany the discriminator @Type() for class-validator to run on the subtype.

How do you implement discriminated union body validation where the DTO shape depends on a type field in NestJS?

How do you implement discriminated union body validation where the DTO shape depends on a type field in NestJS?

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