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
}

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;
}

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
}

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
}

Hint

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

Practice

1. What is the main purpose of using @ValidateNested() in a NestJS DTO?

easy

A. To define default values for nested properties

B. To convert nested objects to plain JavaScript objects

C. To skip validation of nested objects

D. To validate properties of nested objects inside the DTO automatically

Solution

1. Step 1: Understand the role of @ValidateNested()

   This decorator tells NestJS to validate nested objects inside a DTO, not just the top-level properties.

2. Step 2: Compare with other options

   @Type() handles transformation, not validation. Skipping validation or setting defaults is unrelated to @ValidateNested().

3. Final Answer:

   To validate properties of nested objects inside the DTO automatically -> Option D

4. Quick Check:

   @ValidateNested() = validate nested objects [OK]

Hint: Remember: @ValidateNested() triggers nested object validation [OK]

Common Mistakes:

• Confusing @ValidateNested() with @Type()
• Thinking it skips validation
• Assuming it sets default values

2. Which of the following is the correct way to apply nested validation in a DTO property?

easy

A. Use both @ValidateNested() and @Type(() => NestedDto)

B. Use @ValidateNested() only

C. Use @Type(() => NestedDto) only

D. Use @IsString() on the nested property

Solution

1. Step 1: Identify decorators needed for nested validation

   @ValidateNested() triggers validation of nested objects, but @Type() is needed to transform plain objects into class instances.

2. Step 2: Understand why both are required

   Without @Type(), validation won't work properly because the nested object won't be recognized as the correct class type.

3. Final Answer:

   Use both @ValidateNested() and @Type(() => NestedDto) -> Option A

4. Quick Check:

   Nested validation = @ValidateNested() + @Type() [OK]

Hint: Always pair @ValidateNested() with @Type() for nested DTOs [OK]

Common Mistakes:

• Using only one decorator and expecting full validation
• Applying @IsString() on an object property
• Forgetting to import Type from class-transformer

3. Given the following DTOs, what will happen if address.zipCode is missing when validating UserDto?

class AddressDto {
  @IsString()
  street: string;

  @IsString()
  zipCode: string;
}

class UserDto {
  @IsString()
  name: string;

  @ValidateNested()
  @Type(() => AddressDto)
  address: AddressDto;
}

medium

A. Validation will fail because zipCode is missing

B. Validation will pass because zipCode is optional

C. Validation will fail because name is missing

D. Validation will pass but address will be undefined

Solution

1. Step 1: Check required properties in AddressDto

   Both street and zipCode are decorated with @IsString(), so they are required and must be strings.

2. Step 2: Understand nested validation in UserDto

   @ValidateNested() with @Type() ensures address is validated as AddressDto, so missing zipCode causes validation to fail.

3. Final Answer:

   Validation will fail because zipCode is missing -> Option A

4. Quick Check:

   Missing required nested property = validation fails [OK]

Hint: Missing required nested fields cause validation failure [OK]

Common Mistakes:

• Assuming missing nested fields are allowed by default
• Ignoring the effect of @ValidateNested()
• Confusing missing nested fields with missing top-level fields

4. Consider this DTO code snippet:

class ProfileDto {
  @IsString()
  bio: string;
}

class UserDto {
  @IsString()
  username: string;

  @ValidateNested()
  profile: ProfileDto;
}

Why will validation NOT work correctly for the nested profile property?

medium

A. Because @ValidateNested() should be on ProfileDto class

B. Because @IsString() is missing on profile

C. Because @Type(() => ProfileDto) decorator is missing on profile

D. Because bio property is missing @ValidateNested()

Solution

1. Step 1: Identify missing decorator for nested validation

   For nested validation to work, @Type(() => ProfileDto) must be added to the nested property to transform plain objects into class instances.

2. Step 2: Understand why other options are incorrect

   @IsString() is for string properties, not objects. @ValidateNested() belongs on the nested property, not the nested class. bio is a string, no nested validation needed.

3. Final Answer:

   Because @Type(() => ProfileDto) decorator is missing on profile -> Option C

4. Quick Check:

   Missing @Type() breaks nested validation [OK]

Hint: Always add @Type() with @ValidateNested() for nested DTOs [OK]

Common Mistakes:

• Forgetting @Type() on nested properties
• Misplacing @ValidateNested() on nested class
• Confusing property decorators with class decorators

5. You have a CompanyDto with a nested array of EmployeeDto objects. Which is the correct way to validate this nested array properly?

class EmployeeDto {
  @IsString()
  name: string;

  @IsInt()
  age: number;
}

class CompanyDto {
  @IsString()
  companyName: string;

  @ValidateNested({ each: true })
  @Type(() => EmployeeDto)
  employees: EmployeeDto[];
}

hard

A. You must remove { each: true } from @ValidateNested() for arrays

B. This setup correctly validates each employee in the employees array

C. You should use @IsArray() instead of @ValidateNested() for employees

D. You need to add @IsOptional() to employees to validate nested arrays

Solution

1. Step 1: Understand nested array validation

   For arrays of nested objects, @ValidateNested({ each: true }) tells the validator to check each item in the array.

2. Step 2: Confirm use of @Type() for transformation

   @Type(() => EmployeeDto) converts plain objects to EmployeeDto instances, enabling validation on each employee.

3. Final Answer:

   This setup correctly validates each employee in the employees array -> Option B

4. Quick Check:

   Nested arrays need @ValidateNested({ each: true }) + @Type() [OK]

Hint: Use { each: true } with @ValidateNested() for nested arrays [OK]

Common Mistakes:

• Omitting { each: true } for arrays
• Using @IsArray() alone without nested validation
• Forgetting @Type() for nested array items