fix: ValidatedNested with transform not validating

Versions:

• class-transformer: 0.4.0
• class-validator: 0.13.2

Description

Using Transform from class-transformer and ValidateNested of class-validator seems to skip validation of nested fields.

Minimal code-snippet showcasing the problem

import 'reflect-metadata'
import { IsInt, ValidateNested, validateSync } from 'class-validator'
import { plainToClass, Transform, Type } from 'class-transformer'

class Son {
    @IsInt()
    num!: number
}

class Parent {
    @ValidateNested()
    @Type(() => Son)
    @Transform(({ value }) => ({ num: 'a' }))
    son!: Son
}

const parent = plainToClass(Parent, { son: { num: 'abc' } })
console.log('errors: ', validateSync(parent).toString())  // Passing with no validation errors

Expected behavior

Displaying validation error - in this case not passing the isInt.

Actual behavior

Passing validation

[manistra]

For my particular issue I wanted to parse a string into an object and validate it… this works for me:

export class UploadDocumentDto {
  @ValidateNested()
  @Transform(({ value }) => plainToClass(UploadDocument, JSON.parse(value)), {
    toClassOnly: true,
  })
  @Type(() => UploadDocument)
  readonly documents: UploadDocument[];
}
export class UploadDocument {
  @IsNotEmpty()
  @IsString()
  id: string;

  @IsNotEmpty()
  @IsString()
  name: string;
}

[manistra]

validation and transformations work with class instances. try this

class Parent {
    @ValidateNested()
    @Type(() => Son)
    @Transform(({ value }) => {
      // change and return the current instance of `Son` or create new one
       value.num = 'a';
       return value;
    })
    son!: Son
}

Tried

	@Transform(
		({ value }) => {
			value = JSON.parse(value);
			return value;
		},
		{ toClassOnly: true },
	)

No difference

Instead of just: return value try using: return plainToClass(DesiredClassDto, value)

For some reason { toClassOnly: true } alone doesn’t do the trick, and if I’m not wrong for values to be validated they need to be part of a dto class instance.