Object types and input types in NestJS - Mini Project: Build & Apply

NestJS Object Types and Input Types

📖 Scenario: You are building a simple NestJS API to manage a list of books in a library. Each book has a title and an author.We will create object types to represent the book data and input types to handle new book creation requests.

🎯 Goal: Create a NestJS GraphQL object type for Book and an input type for CreateBookInput. Then use these types in a resolver to add new books.

📋 What You'll Learn

Create a GraphQL object type called Book with fields title and author of type string.

Create a GraphQL input type called CreateBookInput with fields title and author of type string.

Create a resolver method called addBook that takes CreateBookInput as an argument and returns a Book.

Use decorators @ObjectType(), @Field(), and @InputType() correctly.

💡 Why This Matters

🌍 Real World

APIs often need to define clear data structures for input and output. NestJS GraphQL uses object types and input types to do this cleanly.

💼 Career

Understanding how to create and use object and input types in NestJS GraphQL is essential for backend developers working with modern APIs.

1

Create the Book object type

Create a class called Book decorated with @ObjectType(). Add two string fields: title and author, each decorated with @Field().

NestJS

import { ObjectType, Field } from '@nestjs/graphql';

// Define the Book object type here
# Your code here

Hint

Use @ObjectType() above the class and @Field() above each property.

2

Create the CreateBookInput input type

Create a class called CreateBookInput decorated with @InputType(). Add two string fields: title and author, each decorated with @Field().

NestJS

import { ObjectType, Field } from '@nestjs/graphql';

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

import { InputType } from '@nestjs/graphql';

// Define the CreateBookInput input type here
# Your code here

Hint

Use @InputType() above the class and @Field() above each property.

3

Create the addBook resolver method

In a resolver class called BooksResolver, create a method called addBook decorated with @Mutation(() => Book). It should take one argument called createBookInput of type CreateBookInput decorated with @Args('createBookInput'). The method should return a new Book object with the same title and author as createBookInput.

NestJS

import { ObjectType, Field } from '@nestjs/graphql';

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

import { InputType, Field } from '@nestjs/graphql';

@InputType()
export class CreateBookInput {
  @Field()
  title: string;

  @Field()
  author: string;
}

import { Resolver, Mutation, Args } from '@nestjs/graphql';

@Resolver()
export class BooksResolver {
  // Create addBook mutation here
  # Your code here
}

Hint

Use @Mutation(() => Book) above the method and @Args('createBookInput') for the argument.

4

Export the resolver and complete setup

Export the BooksResolver class and ensure all imports are correct. This completes the setup for object types and input types in NestJS GraphQL.

NestJS

import { ObjectType, Field } from '@nestjs/graphql';

@ObjectType()
export class Book {
  @Field()
  title: string;

  @Field()
  author: string;
}

import { InputType, Field } from '@nestjs/graphql';

@InputType()
export class CreateBookInput {
  @Field()
  title: string;

  @Field()
  author: string;
}

import { Resolver, Mutation, Args } from '@nestjs/graphql';

@Resolver()
export class BooksResolver {
  @Mutation(() => Book)
  addBook(@Args('createBookInput') createBookInput: CreateBookInput): Book {
    return {
      title: createBookInput.title,
      author: createBookInput.author
    };
  }
}

// Export the resolver class
# Your code here

Hint

Use export { BooksResolver }; at the end of the file.

Practice

1. What is the main purpose of using @ObjectType() in NestJS GraphQL?

easy

A. To define the shape of data returned by queries and mutations

B. To define the shape of data clients send to the server

C. To create database entities automatically

D. To handle HTTP request validation

Solution

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

   This decorator is used to define the structure of data that GraphQL queries and mutations return to clients.

2. Step 2: Differentiate from input types

   @InputType() is used for data clients send, while @ObjectType() is for data sent back to clients.

3. Final Answer:

   To define the shape of data returned by queries and mutations -> Option A

4. Quick Check:

   ObjectType = output data shape [OK]

Hint: ObjectType decorates output data, InputType decorates input data [OK]

Common Mistakes:

• Confusing @ObjectType() with @InputType()
• Thinking @ObjectType() creates database tables
• Assuming @ObjectType() validates HTTP requests

2. Which of the following is the correct way to define an input type in NestJS GraphQL?

easy

A. @ObjectType() class CreateUserInput { name: string; }

B. @InputType() interface CreateUserInput { name: string; }

C. @InputType() class CreateUserInput { @Field() name: string; }

D. @Input() class CreateUserInput { name: string; }

Solution

1. Step 1: Identify correct decorator and syntax

   Input types require the @InputType() decorator and fields must be decorated with @Field() to expose them to GraphQL.

2. Step 2: Check class vs interface usage

   Input types must be classes, not interfaces, to work with decorators in NestJS.

3. Final Answer:

   @InputType() class CreateUserInput { @Field() name: string; } -> Option C

4. Quick Check:

   InputType + @Field + class = correct input type [OK]

Hint: Input types need @InputType() and @Field() on class properties [OK]

Common Mistakes:

• Using @ObjectType() instead of @InputType()
• Using interfaces instead of classes for input types
• Omitting @Field() decorator on fields

3. Given the following code, what will be the GraphQL output type for the user query?

@ObjectType()
class User {
  @Field()
  id: string;

  @Field()
  name: string;
}

@Resolver()
class UserResolver {
  @Query(() => User)
  user() {
    return { id: '1', name: 'Alice' };
  }
}

medium

A. An error because id is not a number

B. A User object with fields id and name

C. A list of User objects

D. Null because no data source is connected

Solution

1. Step 1: Analyze the @ObjectType and @Field decorators

   The User class defines two fields, id and name, both exposed to GraphQL via @Field().

2. Step 2: Check the resolver return and query type

   The user query returns an object matching User shape with id and name strings, matching the GraphQL type.

3. Final Answer:

   A User object with fields id and name -> Option B

4. Quick Check:

   Query returns User object with id and name [OK]

Hint: Return object matches @ObjectType class shape for query output [OK]

Common Mistakes:

• Expecting type errors for string id when GraphQL expects string
• Assuming query returns a list without @Query(() => [User])
• Thinking null is returned without data source

4. Identify the error in this input type definition:

@InputType()
class UpdateUserInput {
  name: string;
  @Field()
  age: number;
}

medium

A. Missing @Field() decorator on the 'name' property

B. Cannot use number type in input types

C. Class must be decorated with @ObjectType() instead

D. Input types cannot have more than one property

Solution

1. Step 1: Check decorators on class properties

   In GraphQL input types, each field must have the @Field() decorator to be exposed.

2. Step 2: Identify missing decorator

   The 'name' property lacks the @Field() decorator, so it won't be part of the GraphQL input type.

3. Final Answer:

   Missing @Field() decorator on the 'name' property -> Option A

4. Quick Check:

   All input fields need @Field() [OK]

Hint: Every input field needs @Field() decorator [OK]

Common Mistakes:

• Forgetting @Field() on some properties
• Confusing @InputType() with @ObjectType()
• Thinking number types are invalid in input types

5. You want to create a mutation to update a user's profile with optional fields. Which approach correctly defines the input type to allow optional fields in NestJS GraphQL?

@InputType()
class UpdateUserInput {
  @Field({ nullable: true })
  name?: string;

  @Field({ nullable: true })
  age?: number;
}

hard

A. Use @ObjectType() instead of @InputType() for mutations

B. Remove '?' and set default values instead of nullable

C. Make all fields required and handle optional logic in resolver

D. Use @Field({ nullable: true }) and optional properties with '?' to allow partial updates

Solution

1. Step 1: Understand optional fields in GraphQL input types

   To allow optional fields, use { nullable: true } in @Field() and mark properties optional with '?'.

2. Step 2: Confirm correct decorator usage

   Use @InputType() for input data in mutations, not @ObjectType().

3. Final Answer:

   Use @Field({ nullable: true }) and optional properties with '?' to allow partial updates -> Option D

4. Quick Check:

   Nullable fields + optional properties = optional input fields [OK]

Hint: Use nullable:true and '?' for optional input fields [OK]

Common Mistakes:

• Using @ObjectType() for input types
• Not marking fields nullable for optional inputs
• Making all fields required and complicating resolver logic