Code-first approach in NestJS - Mini Project: Build & Apply

Building a GraphQL API with NestJS Code-First Approach

📖 Scenario: You are creating a simple GraphQL API for a bookstore. You want to define your data models and GraphQL schema using code, not schema files.

🎯 Goal: Build a NestJS GraphQL API using the code-first approach. You will create a book model, configure GraphQL, write a resolver to fetch books, and complete the module setup.

📋 What You'll Learn

Create a Book class with id, title, and author fields using GraphQL decorators

Add a configuration variable books as an array of book objects

Write a resolver method getBooks that returns the books array

Complete the AppModule to import GraphQLModule with code-first setup and register the resolver

💡 Why This Matters

🌍 Real World

Many modern APIs use GraphQL to allow clients to request exactly the data they need. NestJS's code-first approach lets developers define GraphQL schemas using familiar TypeScript classes and decorators.

💼 Career

Understanding how to build GraphQL APIs with NestJS is valuable for backend developers working on scalable and flexible APIs in Node.js environments.

1

Create the Book model with GraphQL decorators

Create a class called Book with fields id (number), title (string), and author (string). Use @ObjectType() on the class and @Field() on each field from @nestjs/graphql.

NestJS

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

// Define the Book class with GraphQL decorators
// Your code here

Hint

Use @ObjectType() above the class and @Field() above each property. For id, specify the type as Int.

2

Add a books array with sample data

Create a constant array called books with two book objects. Each object should have id, title, and author matching the Book class fields.

NestJS

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

@ObjectType()
export class Book {
  @Field(type => Int)
  id: number;

  @Field()
  title: string;

  @Field()
  author: string;
}

// Create an array called books with two book objects
// Your code here

Hint

Define books as an array of objects matching the Book structure.

3

Create a resolver with a query to get books

Create a class called BooksResolver. Use @Resolver(() => Book) on the class. Add a method getBooks decorated with @Query(() => [Book]) that returns the books array.

NestJS

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

@ObjectType()
export class Book {
  @Field(type => Int)
  id: number;

  @Field()
  title: string;

  @Field()
  author: string;
}

export const books: Book[] = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'The Hobbit', author: 'J.R.R. Tolkien' },
];

// Define BooksResolver class with getBooks method
// Your code here

Hint

Use @Resolver(() => Book) on the class and @Query(() => [Book]) on the method returning books.

4

Complete AppModule with GraphQLModule and resolver

In AppModule, import GraphQLModule from @nestjs/graphql and configure it with autoSchemaFile: true. Add BooksResolver to the providers array.

NestJS

import { ObjectType, Field, Int, Resolver, Query } from '@nestjs/graphql';
import { Module } from '@nestjs/common';
import { GraphQLModule } from '@nestjs/graphql';

@ObjectType()
export class Book {
  @Field(type => Int)
  id: number;

  @Field()
  title: string;

  @Field()
  author: string;
}

export const books: Book[] = [
  { id: 1, title: '1984', author: 'George Orwell' },
  { id: 2, title: 'The Hobbit', author: 'J.R.R. Tolkien' },
];

@Resolver(() => Book)
export class BooksResolver {
  @Query(() => [Book])
  getBooks(): Book[] {
    return books;
  }
}

@Module({
  // Add imports and providers here
  // Your code here
})
export class AppModule {}

Hint

Use GraphQLModule.forRoot({ autoSchemaFile: true }) in imports and add BooksResolver to providers.

Practice

1. What does the code-first approach in NestJS GraphQL primarily involve?

easy

A. Manually writing GraphQL schema files before coding

B. Using REST endpoints instead of GraphQL

C. Writing TypeScript classes and decorators to define GraphQL schema

D. Generating code from existing GraphQL schema files

Solution

1. Step 1: Understand code-first approach basics

   The code-first approach means you write your GraphQL types and resolvers directly in TypeScript code using classes and decorators.

2. Step 2: Compare with other approaches

   Unlike schema-first, you do not write schema files manually; instead, the schema is generated automatically from your code.

3. Final Answer:

   Writing TypeScript classes and decorators to define GraphQL schema -> Option C

4. Quick Check:

   Code-first = Define schema in code [OK]

Hint: Code-first means schema comes from your TypeScript classes [OK]

Common Mistakes:

• Confusing code-first with schema-first approach
• Thinking schema files must be manually written
• Assuming REST is involved in code-first GraphQL

2. Which decorator is used in NestJS code-first approach to mark a class as a GraphQL object type?

easy

A. @ObjectType()

B. @Resolver()

C. @Query()

D. @Injectable()

Solution

1. Step 1: Identify decorator for GraphQL object type

   In code-first, @ObjectType() marks a class as a GraphQL object type to generate schema.

2. Step 2: Differentiate from other decorators

   @Resolver() marks resolver classes, @Query() marks query methods, and @Injectable() is for dependency injection, not schema types.

3. Final Answer:

   @ObjectType() -> Option A

4. Quick Check:

   @ObjectType = GraphQL type class [OK]

Hint: Use @ObjectType() for GraphQL types, not @Resolver() [OK]

Common Mistakes:

• Using @Resolver() instead of @ObjectType() for types
• Confusing @Query() with type decorators
• Thinking @Injectable() defines GraphQL schema

3. Given this code-first resolver snippet, what will be the GraphQL query result for hello()?

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

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

medium

A. "Hello, NestJS!"

B. null

C. Error: Missing return type

D. undefined

Solution

1. Step 1: Analyze the resolver method

   The method hello() returns the string 'Hello, NestJS!'. The @Query decorator specifies the return type as String.

2. Step 2: Understand GraphQL query result

   When queried, the hello field returns the string value wrapped in quotes as a valid GraphQL string response.

3. Final Answer:

   "Hello, NestJS!" -> Option A

4. Quick Check:

   hello() returns string 'Hello, NestJS!' [OK]

Hint: Resolver method return matches GraphQL query result [OK]

Common Mistakes:

• Expecting null or undefined instead of returned string
• Thinking missing return type causes runtime error
• Confusing resolver method with schema definition

4. Identify the error in this code-first resolver snippet:

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

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

medium

A. Method sample() should be async

B. Missing return type in @Query decorator

C. Resolver class missing @ObjectType decorator

D. Return value 123 is invalid in GraphQL

Solution

1. Step 1: Check @Query decorator usage

   The @Query decorator requires a return type function, e.g., @Query(() => Int) or @Query(() => String).

2. Step 2: Identify missing return type

   Here, @Query() is missing the return type, which causes schema generation errors.

3. Final Answer:

   Missing return type in @Query decorator -> Option B

4. Quick Check:

   @Query needs return type function [OK]

Hint: Always specify return type in @Query(() => Type) [OK]

Common Mistakes:

• Omitting return type in @Query decorator
• Adding @ObjectType to resolver class incorrectly
• Assuming async is mandatory for resolver methods

5. You want to create a GraphQL object type with a field that is optional and nullable using the code-first approach. Which is the correct way to define the field in your class?

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

@ObjectType()
export class User {
  @Field(() => String, { nullable: true })
  nickname?: string;
}

What does this code do?

hard

A. Ignores the nickname field in the generated schema

B. Causes a schema error because optional fields are not allowed

C. Makes nickname required and non-nullable in the schema

D. Defines a User type with an optional nullable nickname field in GraphQL schema

Solution

1. Step 1: Understand @Field decorator options

   The @Field decorator with { nullable: true } marks the field as nullable in GraphQL schema, allowing null values.

2. Step 2: Check TypeScript optional property

   Using ? in nickname?: string means the property is optional in TypeScript, matching nullable in GraphQL.

3. Step 3: Confirm schema generation behavior

   This combination correctly generates a GraphQL field that can be omitted or set to null without errors.

4. Final Answer:

   Defines a User type with an optional nullable nickname field in GraphQL schema -> Option D

5. Quick Check:

   nullable + optional = optional nullable field [OK]

Hint: Use @Field({ nullable: true }) and ? for optional nullable fields [OK]

Common Mistakes:

• Omitting nullable option for optional fields
• Confusing optional in TypeScript with required in GraphQL
• Expecting errors for optional nullable fields