Nested DTO validation in NestJS - Mini Project: Build & Apply

Nested DTO Validation in NestJS

📖 Scenario: You are building a simple API for a bookstore. Each book has a title and an author. The author itself has a name and an email. You want to make sure that when someone sends data to create a book, both the book and the author data are checked carefully.

🎯 Goal: Create nested DTO classes for Book and Author with validation rules. Then use these DTOs in a controller to validate incoming data automatically.

📋 What You'll Learn

Create a DTO class called AuthorDto with name and email properties.

Add validation decorators to AuthorDto: name must be a non-empty string, email must be a valid email.

Create a DTO class called BookDto with title and author properties.

Add validation decorators to BookDto: title must be a non-empty string, author must be a nested AuthorDto.

Use @ValidateNested() and @Type(() => AuthorDto) decorators to enable nested validation.

Create a simple controller method that accepts BookDto and validates the input automatically.

💡 Why This Matters

🌍 Real World

APIs often receive complex data with nested objects. Validating nested DTOs ensures data integrity and prevents errors early.

💼 Career

Backend developers use nested DTO validation in NestJS to build robust APIs that handle complex data safely and clearly.

1

Create the AuthorDto class with properties

Create a class called AuthorDto with two properties: name and email. Both should be strings.

NestJS

class AuthorDto {
  // Your code here
}

Need a hint?

Define a class with two string properties named exactly name and email.

2

Add validation decorators to AuthorDto

Add @IsString() and @IsNotEmpty() decorators to name, and @IsEmail() decorator to email in the AuthorDto class. Import these decorators from class-validator.

NestJS

import { IsString, IsNotEmpty, IsEmail } from 'class-validator';

class AuthorDto {
  // Add decorators here
  name: string;
  email: string;
}

Need a hint?

Use @IsString() and @IsNotEmpty() on name, and @IsEmail() on email.

3

Create BookDto with nested AuthorDto and validation

Create a class called BookDto with a title property as a non-empty string and an author property of type AuthorDto. Use @IsString() and @IsNotEmpty() on title. Use @ValidateNested() and @Type(() => AuthorDto) on author. Import ValidateNested from class-validator and Type from class-transformer.

NestJS

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

class BookDto {
  // Add properties and decorators here
}

Need a hint?

Use @ValidateNested() and @Type(() => AuthorDto) on the author property to enable nested validation.

4

Use BookDto in a controller method with validation pipe

Create a NestJS controller class called BooksController with a createBook method. This method should accept a BookDto parameter decorated with @Body() and use ValidationPipe to validate the input automatically. Import necessary decorators and pipes from @nestjs/common.

NestJS

import { Controller, Post, Body, ValidationPipe } from '@nestjs/common';

@Controller('books')
export class BooksController {
  // Add createBook method here
}

Need a hint?

Use @Body(new ValidationPipe()) to validate the incoming BookDto automatically.