Code-first approach in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Code-first approach in NestJS GraphQL?

It means you write your TypeScript classes and decorators first, then NestJS generates the GraphQL schema from your code automatically.

beginner

How do you define a GraphQL object type using the Code-first approach in NestJS?

You create a TypeScript class and decorate it with @ObjectType(), then decorate its fields with @Field() to expose them in the GraphQL schema.

beginner

Why is the Code-first approach helpful for beginners?

Because you work directly with familiar TypeScript code and decorators, avoiding manual schema writing and reducing errors.

beginner

Which decorator is used to mark a method as a GraphQL query in the Code-first approach?

The @Query() decorator is used on a method inside a resolver class to define a GraphQL query.

intermediate

How does NestJS generate the GraphQL schema in the Code-first approach?

NestJS scans your decorated classes and methods, then builds the schema automatically at runtime based on your code structure.

In NestJS Code-first approach, which decorator marks a class as a GraphQL object type?

A @Resolver()

B @ObjectType()

C @InputType()

D @Field()

The @ObjectType() decorator marks a class as a GraphQL object type in the Code-first approach.

Which decorator is used to expose a class property as a GraphQL field?

A @Field()

B @Query()

C @Mutation()

D @ObjectType()

The @Field() decorator exposes a class property as a GraphQL field.

What does the Code-first approach generate automatically from your code?

A Database migrations

B REST API endpoints

C GraphQL schema

D CSS styles

The Code-first approach automatically generates the GraphQL schema from your decorated TypeScript code.

Which decorator defines a GraphQL query method in a resolver?

A @Query()

B @Mutation()

C @Field()

D @ObjectType()

The @Query() decorator marks a method as a GraphQL query.

Why might developers prefer Code-first over Schema-first in NestJS?

A Because it requires writing schema files manually

B Because it only works with REST APIs

C Because it does not support GraphQL

D Because it uses decorators and TypeScript classes, making code easier to maintain

Code-first uses decorators and TypeScript classes, which many find easier to maintain and less error-prone.

Explain how the Code-first approach works in NestJS for building a GraphQL API.

Think about how your code becomes the schema.

List the main decorators used in the Code-first approach and their purpose.

Focus on decorators that define schema and operations.

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