Code-first approach in NestJS

Introduction

The code-first approach lets you create your GraphQL API by writing code first. This means you define your data and logic in code, and the GraphQL schema is generated automatically.

When you want to build a GraphQL API quickly without writing schema files manually.

When you prefer to keep your API definitions close to your business logic code.

When you want automatic type safety and validation from your code classes.

When you want to use decorators to describe your GraphQL types and fields.

When you want to avoid syncing separate schema files with your code.

Syntax

NestJS

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

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

  @Field()
  name: string;
}

@ObjectType marks a class as a GraphQL type.

@Field marks class properties as GraphQL fields and can specify their types.

Examples

This defines a simple User type with two string fields.

NestJS

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

@ObjectType()
export class User {
  @Field()
  username: string;

  @Field()
  email: string;
}

This defines a Post type with an integer id, a title, and an optional content field.

NestJS

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

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

  @Field()
  title: string;

  @Field({ nullable: true })
  content?: string;
}

Sample Program

This example shows a Book type and a resolver that returns a list of books. The GraphQL schema is generated from the Book class and the resolver methods.

NestJS

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

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

  @Field()
  title: string;

  @Field()
  author: string;
}

@Resolver(of => Book)
export class BookResolver {
  private books: Book[] = [
    { id: 1, title: '1984', author: 'George Orwell' },
    { id: 2, title: 'The Hobbit', author: 'J.R.R. Tolkien' },
  ];

  @Query(returns => [Book])
  books() {
    return this.books;
  }
}

Important Notes

Use decorators like @ObjectType and @Field to define your GraphQL types in code.

The code-first approach automatically creates the schema, so you don't write .graphql files.

This approach keeps your API definitions and business logic together, making maintenance easier.

Summary

The code-first approach means writing your GraphQL types and resolvers in code first.

Decorators mark classes and fields to generate the GraphQL schema automatically.

This method is great for quick development and keeping code organized.

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