Why Nested DTO validation in NestJS? - Purpose & Use Cases

The Big Idea

Discover how to stop chasing bugs in complex data and let your code validate itself!

The Scenario

Imagine building a form where users submit complex data with multiple layers, like an order containing customer info and a list of products.

You try to check each piece manually in your code to ensure everything is correct.

The Problem

Manually checking nested data is slow and messy.

You might forget to validate some parts or write repetitive code.

Errors become hard to track and fix, making your app unreliable.

The Solution

Nested DTO validation lets you define clear rules for each data layer.

The framework automatically checks all nested parts, keeping your code clean and consistent.

Before vs After

✗ Before

if (!data.customer || !data.customer.email) { throw new Error('Invalid customer'); } if (!Array.isArray(data.products)) { throw new Error('Products missing'); }

✓ After

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

class CustomerDto {
  @IsEmail()
  email: string;
}

class ProductDto {
  @IsString()
  name: string;
}

class OrderDto {
  @ValidateNested()
  @Type(() => CustomerDto)
  customer: CustomerDto;

  @ValidateNested({ each: true })
  @Type(() => ProductDto)
  products: ProductDto[];
}

What It Enables

You can trust your app to catch invalid nested data automatically, making development faster and safer.

Real Life Example

When a user submits a signup form with address details inside their profile, nested DTO validation ensures every field is correct before saving.

Key Takeaways

Manual nested data checks are error-prone and repetitive.

Nested DTO validation automates and organizes these checks.

This leads to cleaner code and more reliable apps.

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