Resolvers in NestJS - Mini Project: Build & Apply

Building a Simple GraphQL Resolver in NestJS

📖 Scenario: You are creating a small NestJS GraphQL API for a bookstore. You want to provide a way for clients to get book details by their ID.

🎯 Goal: Build a GraphQL resolver in NestJS that returns a book object with id, title, and author fields when queried by id.

📋 What You'll Learn

Create a simple array of book objects with exact fields and values

Add a variable to hold the book ID to search for

Write a resolver method that returns the book matching the given ID

Complete the resolver class with the proper decorator and export

💡 Why This Matters

🌍 Real World

GraphQL resolvers in NestJS are used to handle client queries and return data from databases or other sources.

💼 Career

Understanding resolvers is essential for backend developers working with NestJS and GraphQL APIs.

1

Create the books data array

Create a constant array called books with these exact objects: { id: '1', title: '1984', author: 'George Orwell' }, { id: '2', title: 'Brave New World', author: 'Aldous Huxley' }, and { id: '3', title: 'Fahrenheit 451', author: 'Ray Bradbury' }.

NestJS

// Create the books array with 3 book objects
// Your code here

Hint

Use const books = [ ... ] with exact objects inside.

2

Add the book ID variable

Create a constant called bookId and set it to the string '2' to represent the book to find.

NestJS

const books = [
  { id: '1', title: '1984', author: 'George Orwell' },
  { id: '2', title: 'Brave New World', author: 'Aldous Huxley' },
  { id: '3', title: 'Fahrenheit 451', author: 'Ray Bradbury' },
];

// Create a constant bookId with value '2'
// Your code here

Hint

Use const bookId = '2'; exactly.

3

Write the resolver method to find the book

Create a method called getBookById that returns the book object from books where id matches bookId. Use find on the books array.

NestJS

const books = [
  { id: '1', title: '1984', author: 'George Orwell' },
  { id: '2', title: 'Brave New World', author: 'Aldous Huxley' },
  { id: '3', title: 'Fahrenheit 451', author: 'Ray Bradbury' },
];

const bookId = '2';

// Create a function getBookById that returns the book with id === bookId
// Your code here

Hint

Use books.find(book => book.id === bookId) inside the function.

4

Complete the resolver class with decorator

Create a class called BookResolver decorated with @Resolver(). Inside it, add a method book decorated with @Query(() => Object) that calls and returns getBookById(). Export the class.

NestJS

const books = [
  { id: '1', title: '1984', author: 'George Orwell' },
  { id: '2', title: 'Brave New World', author: 'Aldous Huxley' },
  { id: '3', title: 'Fahrenheit 451', author: 'Ray Bradbury' },
];

const bookId = '2';

function getBookById() {
  return books.find(book => book.id === bookId);
}

// Create and export BookResolver class with @Resolver() decorator
// Add a @Query(() => Object) method named book that returns getBookById()
// Your code here

Hint

Use @Resolver() on the class and @Query(() => Object) on the method.

Practice

1. What is the main purpose of a @Resolver in a NestJS GraphQL application?

easy

A. To handle GraphQL queries and mutations by connecting them to data or actions

B. To define database schemas for the application

C. To manage HTTP routes and middleware

D. To style the frontend components

Solution

1. Step 1: Understand the role of @Resolver

   The @Resolver decorator marks a class that handles GraphQL queries and mutations, linking them to the app's data or business logic.

2. Step 2: Differentiate from other roles

   It does not define database schemas, manage HTTP routes, or style frontend components, which are handled by other parts of the app.

3. Final Answer:

   To handle GraphQL queries and mutations by connecting them to data or actions -> Option A

4. Quick Check:

   Resolver purpose = handle GraphQL queries/mutations [OK]

Hint: Resolvers link GraphQL requests to your app's data/actions [OK]

Common Mistakes:

• Confusing resolvers with database schema definitions
• Thinking resolvers manage HTTP routes
• Assuming resolvers handle frontend styling

2. Which of the following is the correct way to define a GraphQL query resolver method in NestJS?

easy

A. @Controller()
getHello() {
return 'Hello World';
}

B. @Mutation(() => String)
getHello() {
return 'Hello World';
}

C. @Query(() => String)
getHello() {
return 'Hello World';
}

D. @Resolver(() => String)
getHello() {
return 'Hello World';
}

Solution

1. Step 1: Identify the decorator for queries

   In NestJS GraphQL, @Query decorator is used to define query resolver methods.

2. Step 2: Check method syntax

   The method returns a string and is decorated with @Query(() => String), which is correct syntax.

3. Final Answer:

   @Query(() => String) getHello() { return 'Hello World'; } -> Option C

4. Quick Check:

   Query method uses @Query decorator [OK]

Hint: Use @Query for queries, @Mutation for mutations [OK]

Common Mistakes:

• Using @Mutation instead of @Query for queries
• Using @Resolver on methods instead of classes
• Using @Controller which is for REST, not GraphQL

3. Given this resolver code, what will be the output when the hello query is called?

@Resolver()
export class HelloResolver {
  @Query(() => String)
  hello() {
    return 'Hi NestJS';
  }
}

medium

A. Error: Missing return type

B. "Hello World"

C. null

D. "Hi NestJS"

Solution

1. Step 1: Analyze the resolver method

   The hello method is decorated with @Query(() => String) and returns the string 'Hi NestJS'.

2. Step 2: Determine the query output

   When the hello query is called, it returns the string exactly as coded: 'Hi NestJS'.

3. Final Answer:

   "Hi NestJS" -> Option D

4. Quick Check:

   Query returns 'Hi NestJS' string [OK]

Hint: Return value inside @Query method is the output [OK]

Common Mistakes:

• Expecting 'Hello World' instead of 'Hi NestJS'
• Thinking a return type error occurs
• Assuming null is returned by default

4. Identify the error in this NestJS resolver code:

@Resolver()
export class SampleResolver {
  @Query(() => String)
  sample() {
    return 123;
  }
}

medium

A. The method returns a number but the return type is declared as String

B. Missing @Mutation decorator

C. The class is missing @Injectable decorator

D. The method name should be capitalized

Solution

1. Step 1: Check the return type annotation

   The @Query(() => String) decorator declares the return type as String.

2. Step 2: Check the actual return value

   The method returns the number 123, which conflicts with the declared String return type.

3. Final Answer:

   The method returns a number but the return type is declared as String -> Option A

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match decorator type [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking @Mutation is needed for queries
• Assuming @Injectable is required on resolvers

5. You want to create a resolver that returns a list of users filtered by an optional age argument. Which code snippet correctly implements this in NestJS?

hard

A. @Resolver()
export class UserResolver {
@Mutation(() => [User])
users(@Args('age') age: number) {
return users.filter(u => u.age === age);
}
}

B. @Resolver()
export class UserResolver {
@Query(() => [User])
users(@Args('age', { nullable: true }) age?: number) {
return age ? users.filter(u => u.age === age) : users;
}
}

C. @Resolver()
export class UserResolver {
@Query(() => User)
users(@Args('age') age: number) {
return users.find(u => u.age === age);
}
}

D. @Resolver()
export class UserResolver {
@Query(() => [User])
users() {
return users;
}
}

Solution

1. Step 1: Check the query decorator and return type

   The method uses @Query(() => [User]) which correctly indicates it returns a list of users.

2. Step 2: Verify optional argument handling

   The @Args('age', { nullable: true }) age?: number allows the age argument to be optional, filtering users if provided, or returning all users otherwise.

3. Step 3: Confirm filtering logic

   The method filters the users array by age if age is given, otherwise returns all users, matching the requirement.

4. Final Answer:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } -> Option B

5. Quick Check:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } [OK]

Hint: Use nullable args for optional filters in @Query [OK]

Common Mistakes:

• Using @Mutation instead of @Query for fetching data
• Returning single User instead of list
• Not making argument optional when needed