Object types and input types in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Object Type in NestJS GraphQL?

An Object Type defines the shape of data that the GraphQL API returns. It is a class decorated with @ObjectType() that describes fields and their types for the API response.

beginner

What is an Input Type in NestJS GraphQL and how is it used?

An Input Type is a class decorated with @InputType() used to define the shape of data sent by clients to the API, such as arguments for mutations or queries.

beginner

How do you define a simple Object Type with two fields: id (number) and name (string)?

Use a class with @ObjectType() and decorate fields with @Field(). Example:
@ObjectType()
class User {
  @Field()
  id: number;

  @Field()
  name: string;
}

intermediate

Why should Input Types be separate from Object Types in NestJS GraphQL?

Separating Input Types from Object Types helps keep input validation clear and prevents exposing internal fields in API responses. It also avoids circular references.

intermediate

How do you reuse fields between Object Types and Input Types in NestJS?

You can create a base class with shared fields and extend it in both @ObjectType() and @InputType() classes to avoid repeating code.

Which decorator is used to define an Object Type in NestJS GraphQL?

A @ObjectType()

B @InputType()

C @Entity()

D @Controller()

The @ObjectType() decorator marks a class as a GraphQL Object Type for API responses.

What decorator marks a class as an Input Type in NestJS GraphQL?

A @Field()

B @Input()

C @ArgsType()

D @InputType()

The @InputType() decorator defines a class used for input data in GraphQL queries or mutations.

Why should Input Types and Object Types be separate in NestJS GraphQL?

A To improve database performance

B To reduce server memory usage

C To avoid exposing internal fields and circular references

D To enable REST API compatibility

Separating them helps keep input validation clear and prevents exposing internal data or circular dependencies.

Which decorator is used to define fields inside Object or Input Types?

A @Column()

B @Field()

C @Property()

D @Param()

@Field() marks a property as a GraphQL field in Object or Input Types.

How can you reuse fields between Object Types and Input Types in NestJS?

A By using inheritance with a shared base class

B By copying code manually

C By using interfaces only

D By using decorators only

Inheritance with a shared base class allows reusing fields in both Object and Input Types.

Explain the difference between Object Types and Input Types in NestJS GraphQL and why both are important.

Think about what data the server sends versus what it receives.

Describe how to create a reusable set of fields for both Object Types and Input Types in NestJS.

Consider how inheritance works in classes.

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