Queries and mutations in NestJS - Mini Project: Build & Apply

Building Queries and Mutations in NestJS GraphQL

📖 Scenario: You are creating a simple NestJS GraphQL API for a bookstore. You want to let users get book details and add new books.

🎯 Goal: Build a NestJS GraphQL resolver with a query to get a book by its id and a mutation to add a new book with title and author.

📋 What You'll Learn

Create a Book class with id, title, and author fields

Create a query called getBook that takes an id argument and returns the matching Book

Create a mutation called addBook that takes title and author arguments and returns the new Book

Use an in-memory array to store books

💡 Why This Matters

🌍 Real World

Building APIs with NestJS and GraphQL is common for modern web apps that need flexible data fetching and updating.

💼 Career

Understanding queries and mutations in NestJS GraphQL is essential for backend developers working with GraphQL APIs.

1

Create the Book class and initial books array

Create a class called Book with id (number), title (string), and author (string) fields. Then create a variable called books as an array with one book: { id: 1, title: '1984', author: 'George Orwell' }.

NestJS

# Create the Book class and books array here
# Your code here

Need a hint?

Define a TypeScript class with the three fields. Then create an array with one object matching the class.

2

Add a query method to get a book by id

Add a method called getBook inside a resolver class BooksResolver. It should take an id argument of type number and return the book from books with that id. Use @Query(() => Book) decorator on the method.

NestJS

class Book {
  id: number;
  title: string;
  author: string;
}

const books: Book[] = [
  { id: 1, title: '1984', author: 'George Orwell' }
];

// Add BooksResolver class with getBook query method here
# Your code here

Need a hint?

Use @Resolver and @Query decorators. Use @Args to get the id argument.

3

Add a mutation method to add a new book

Inside the BooksResolver class, add a method called addBook with title and author string arguments. It should create a new Book with a unique id, add it to books, and return the new book. Use @Mutation(() => Book) decorator on the method.

NestJS

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

class Book {
  id: number;
  title: string;
  author: string;
}

const books: Book[] = [
  { id: 1, title: '1984', author: 'George Orwell' }
];

@Resolver(() => Book)
export class BooksResolver {
  @Query(() => Book, { nullable: true })
  getBook(@Args('id', { type: () => Int }) id: number): Book | undefined {
    return books.find(book => book.id === id);
  }

  // Add addBook mutation method here
# Your code here
}

Need a hint?

Use @Mutation decorator. Create a new book object with a new id and add it to the books array.

4

Complete the resolver export and imports

Add the necessary imports from @nestjs/graphql at the top: Resolver, Query, Args, Int, and Mutation. Export the BooksResolver class so it can be used in the NestJS module.

NestJS

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

// Ensure BooksResolver is exported
# Your code here

Need a hint?

Make sure all decorators are imported and the resolver class is exported.