fix: ValidateNested do not pay attention of empty array as an item of an array

[mtnt]

Description

I tried to validate some property expected as an array of objects. And ValidateNested worked ok with all invalid data except an empty array (fulfilled array was handled ok).

Minimal code-snippet showcasing the problem

import { IsOptional, IsNumber, ArrayNotEmpty, ValidateNested, IsBoolean } from 'class-validator';
import { Type } from 'class-transformer';

export class OneDTO {
  @ArrayNotEmpty({ always: true})
  @Type(() => TwoDTO)
  @ValidateNested({ always: true })
  data!: TwoDTO[];
}

class TwoDTO {
  @IsNumber({}, { always: true})
  id!: number;
}

Expected behavior

• { data: 11 } - failed
• { data: true } - failed
• { data: [] } - failed
• { data: [1] } - failed
• { data: [[]] } - failed
• { data: [[1]] } - failed
• { data: [true] } - failed
• { data: [{}] } - failed
• { data: [{id: ''}] } - failed
• { data: [{id: 1}] } - passed

Actual behavior

• { data: 11 } - failed
• { data: true } - failed
• { data: [] } - failed
• { data: [1] } - failed
• { data: [[]] } - passed <<<<<<<<
• { data: [[1]] } - failed
• { data: [true] } - failed
• { data: [{}] } - failed
• { data: [{id: ''}] } - failed
• { data: [{id: 1}] } - passed

[emir-gradient]

I think that @ValidateNested() is overwhelmed in a way that it has one responsibility too much.

Being able to handle this correctly:

class MySubClass {
      @MinLength(5)
      name: string;
    }

    class MyClass {
      @Contains('hello')
      title: string;

      @ValidateNested()
      mySubClass: MySubClass;

      @ValidateNested()
      mySubClasses: MySubClass[];

      @ValidateNested()
      mySubSubClasses: MySubClass[][];

      @ValidateNested()
      mySubSubSubClasses: MySubClass[][][];
    }

makes it unable to identify empty array as the problem. New empty array, for this validator, means just one more level in the main array, and not wrong object.

I think that probably best long-term solution (to not make breaking changes in sooner period for Users) is to make @ValidateNested() deprecated, and split it into two validators, @ValidateNestedObject() that would accept nested objects only (non array-like ones), and @ValidateNestedArray() that would have one additional mandatory parameter depth or arrayDimension where correct array dimension would be written by User.

Of course, the issue is redundant implementation, and that all Users can not upgrade to version of class-validator which gets @ValidateNested() removed from the library without completely removing it from their projects.

On the other side, to preserve @ValidateNested(), solution that I would say is not so nice, but might be acceptable, is to extend ValidationOptions interface with the optional nestedArrayDimension attribute, and if it is inserted by User, it would be respected, but if it is not, @ValidateNested() would behave as it currently does.

Finally, a an User that utilizes class-validator in many of my projects, I am very interested to contribute to it and try to support it to become even better.