Resolvers in NestJS - Step-by-Step Execution

Concept Flow - Resolvers

Client sends GraphQL query

↓

GraphQL Module receives query

↓

Resolver method matched to query

↓

Resolver executes business logic

↓

Resolver returns data

↓

GraphQL Module sends response to client

Resolvers receive GraphQL queries, run the matching method, and return data back to the client.

Execution Sample

NestJS

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

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

This resolver responds to the 'sayHello' query by returning a greeting string.

Execution Table

Step	Action	Input	Resolver Method	Output
1	Receive query	{ sayHello }	N/A	N/A
2	Match query to resolver	sayHello	sayHello()	N/A
3	Execute resolver method	N/A	sayHello()	'Hello, world!'
4	Send response	N/A	N/A	{ "sayHello": "Hello, world!" }

💡 Query fully resolved and response sent to client.

Variable Tracker

Variable	Start	After Step 3	Final
query	N/A	{ sayHello }	{ sayHello }
resolverMethod	N/A	sayHello()	sayHello()
result	N/A	N/A	'Hello, world!'

Key Moments - 2 Insights

Why does the resolver method name match the query name?

The GraphQL query name must match the resolver method decorated with @Query to connect the request to the correct function, as shown in step 2 of the execution table.

What happens if the resolver method does not return data?

If the resolver returns nothing or undefined, the client will receive null or an error. Step 3 shows the resolver returning a string, which is required for a successful response.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 3?

A { sayHello }

B N/A

C 'Hello, world!'

D null

💡 Hint

Check the 'Output' column at step 3 in the execution table.

At which step does the resolver method get matched to the query?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column describing matching in the execution table.

If the resolver method returned null, how would the final output change?

A Output would be an error at step 2

B Output would be { 'sayHello': null }

C Output would be { 'sayHello': 'Hello, world!' }

D No response would be sent

💡 Hint

Refer to the 'Output' column and understand what happens when resolver returns null.

Concept Snapshot

Resolvers connect GraphQL queries to methods.
Use @Resolver() class and @Query() method decorators.
Method name matches query name.
Return data from method to send response.
NestJS handles query routing automatically.

Full Transcript

Resolvers in NestJS handle GraphQL queries by matching the query name to a method decorated with @Query inside a class decorated with @Resolver. When a client sends a query, the GraphQL module finds the matching resolver method, executes it, and returns the data. For example, a 'sayHello' query calls the 'sayHello' method, which returns 'Hello, world!'. The response is then sent back to the client. This flow ensures queries get the correct data by connecting query names to resolver methods.

Practice

1. What is the main purpose of a @Resolver in a NestJS GraphQL application?

easy

A. To handle GraphQL queries and mutations by connecting them to data or actions

B. To define database schemas for the application

C. To manage HTTP routes and middleware

D. To style the frontend components

Solution

1. Step 1: Understand the role of @Resolver

   The @Resolver decorator marks a class that handles GraphQL queries and mutations, linking them to the app's data or business logic.

2. Step 2: Differentiate from other roles

   It does not define database schemas, manage HTTP routes, or style frontend components, which are handled by other parts of the app.

3. Final Answer:

   To handle GraphQL queries and mutations by connecting them to data or actions -> Option A

4. Quick Check:

   Resolver purpose = handle GraphQL queries/mutations [OK]

Hint: Resolvers link GraphQL requests to your app's data/actions [OK]

Common Mistakes:

• Confusing resolvers with database schema definitions
• Thinking resolvers manage HTTP routes
• Assuming resolvers handle frontend styling

2. Which of the following is the correct way to define a GraphQL query resolver method in NestJS?

easy

A. @Controller()
getHello() {
return 'Hello World';
}

B. @Mutation(() => String)
getHello() {
return 'Hello World';
}

C. @Query(() => String)
getHello() {
return 'Hello World';
}

D. @Resolver(() => String)
getHello() {
return 'Hello World';
}

Solution

1. Step 1: Identify the decorator for queries

   In NestJS GraphQL, @Query decorator is used to define query resolver methods.

2. Step 2: Check method syntax

   The method returns a string and is decorated with @Query(() => String), which is correct syntax.

3. Final Answer:

   @Query(() => String) getHello() { return 'Hello World'; } -> Option C

4. Quick Check:

   Query method uses @Query decorator [OK]

Hint: Use @Query for queries, @Mutation for mutations [OK]

Common Mistakes:

• Using @Mutation instead of @Query for queries
• Using @Resolver on methods instead of classes
• Using @Controller which is for REST, not GraphQL

3. Given this resolver code, what will be the output when the hello query is called?

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

medium

A. Error: Missing return type

B. "Hello World"

C. null

D. "Hi NestJS"

Solution

1. Step 1: Analyze the resolver method

   The hello method is decorated with @Query(() => String) and returns the string 'Hi NestJS'.

2. Step 2: Determine the query output

   When the hello query is called, it returns the string exactly as coded: 'Hi NestJS'.

3. Final Answer:

   "Hi NestJS" -> Option D

4. Quick Check:

   Query returns 'Hi NestJS' string [OK]

Hint: Return value inside @Query method is the output [OK]

Common Mistakes:

• Expecting 'Hello World' instead of 'Hi NestJS'
• Thinking a return type error occurs
• Assuming null is returned by default

4. Identify the error in this NestJS resolver code:

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

medium

A. The method returns a number but the return type is declared as String

B. Missing @Mutation decorator

C. The class is missing @Injectable decorator

D. The method name should be capitalized

Solution

1. Step 1: Check the return type annotation

   The @Query(() => String) decorator declares the return type as String.

2. Step 2: Check the actual return value

   The method returns the number 123, which conflicts with the declared String return type.

3. Final Answer:

   The method returns a number but the return type is declared as String -> Option A

4. Quick Check:

   Return type mismatch causes error [OK]

Hint: Return type must match decorator type [OK]

Common Mistakes:

• Ignoring return type mismatch
• Thinking @Mutation is needed for queries
• Assuming @Injectable is required on resolvers

5. You want to create a resolver that returns a list of users filtered by an optional age argument. Which code snippet correctly implements this in NestJS?

hard

A. @Resolver()
export class UserResolver {
@Mutation(() => [User])
users(@Args('age') age: number) {
return users.filter(u => u.age === age);
}
}

B. @Resolver()
export class UserResolver {
@Query(() => [User])
users(@Args('age', { nullable: true }) age?: number) {
return age ? users.filter(u => u.age === age) : users;
}
}

C. @Resolver()
export class UserResolver {
@Query(() => User)
users(@Args('age') age: number) {
return users.find(u => u.age === age);
}
}

D. @Resolver()
export class UserResolver {
@Query(() => [User])
users() {
return users;
}
}

Solution

1. Step 1: Check the query decorator and return type

   The method uses @Query(() => [User]) which correctly indicates it returns a list of users.

2. Step 2: Verify optional argument handling

   The @Args('age', { nullable: true }) age?: number allows the age argument to be optional, filtering users if provided, or returning all users otherwise.

3. Step 3: Confirm filtering logic

   The method filters the users array by age if age is given, otherwise returns all users, matching the requirement.

4. Final Answer:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } -> Option B

5. Quick Check:

   @Resolver() export class UserResolver { @Query(() => [User]) users(@Args('age', { nullable: true }) age?: number) { return age ? users.filter(u => u.age === age) : users; } } [OK]

Hint: Use nullable args for optional filters in @Query [OK]

Common Mistakes:

• Using @Mutation instead of @Query for fetching data
• Returning single User instead of list
• Not making argument optional when needed