Today I have for you a quick and short article. Maybe it will help someone. I'm using class-validator for request validation in NestJS really often. A few days ago I needed to validate a nested object. Quick look to the class-validator validation:

If your object contains nested objects and you want the validator to perform their validation too, then you need to use the @ValidateNested() decorator:

import { ValidateNested } from 'class-validator';

export class Post {
  @ValidateNested()
  user: User;
}

But for some reason It doesn't work in NestJS! Here is an easy solution. Install class-transformer package, if you haven't done it yet. Then import @Type() decorator, and declare the type of validating object with it. Check this out:

import { ValidateNested } from 'class-validator';
import { Type } from 'class-transformer';

export class Post {
  @ValidateNested()
  @Type(() => User)
  user: User;
}

Now our NestJS application will validate User object correctly. If you need to validate an array of objects, use each: true:

export class User {
  @ValidateNested({ each: true })
  @Type(() => Post)
  posts: Post[];
}

Hope it will be useful for you! Cheers!