Object types and input types in NestJS

Introduction

Object types and input types help define the shape of data in NestJS GraphQL. They make sure data sent and received is clear and correct.

When creating GraphQL queries that return data objects.

When defining the structure of data clients can send to the server.

When you want to validate and type-check data in GraphQL resolvers.

When building APIs that need clear input and output data formats.

Syntax

NestJS

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

@ObjectType()
export class ExampleObject {
  @Field()
  propertyName: string;
}

@InputType()
export class ExampleInput {
  @Field()
  inputProperty: string;
}

@ObjectType() marks a class as a GraphQL object type for outputs.

@InputType() marks a class as a GraphQL input type for inputs.

Examples

This defines a User object type with id and name fields for GraphQL responses.

NestJS

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

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

  @Field()
  name: string;
}

This defines an input type for creating a user, specifying the data clients must send.

NestJS

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

@InputType()
export class CreateUserInput {
  @Field()
  name: string;

  @Field()
  email: string;
}

Sample Program

This example shows a simple NestJS GraphQL resolver with an object type Book and an input type CreateBookInput. You can query all books or add a new book using the input type.

NestJS

import { Resolver, Query, Mutation, Args } from '@nestjs/graphql';
import { ObjectType, Field, InputType } from '@nestjs/graphql';

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

  @Field()
  author: string;
}

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

  @Field()
  author: string;
}

@Resolver()
export class BookResolver {
  private books: Book[] = [];

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

  @Mutation(() => Book)
  addBook(@Args('input') input: CreateBookInput): Book {
    const newBook = new Book();
    newBook.title = input.title;
    newBook.author = input.author;
    this.books.push(newBook);
    return newBook;
  }
}

Important Notes

Always decorate each property with @Field() to expose it in GraphQL schema.

Input types are used for data coming into the server, object types for data going out.

You can reuse object types as input types if needed, but separate classes keep things clear.

Summary

Object types define the shape of data returned by GraphQL queries and mutations.

Input types define the shape of data clients send to the server.

Use decorators @ObjectType() and @InputType() to create these types in NestJS.

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