Queries and mutations in NestJS - Step-by-Step Execution

Concept Flow - Queries and mutations

Client sends GraphQL request

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NestJS GraphQL Resolver

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Query Handler

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Fetch data

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Return response to client

The client sends a GraphQL request which NestJS routes to either a query or mutation handler. Queries fetch data, mutations change data, then responses return to the client.

Execution Sample

NestJS

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

@Resolver()
export class SampleResolver {
  @Query(() => String)
  hello() { return 'Hello World'; }

  @Mutation(() => String)
  updateMessage(@Args('newMsg') newMsg: string) { return newMsg; }
}

Defines a simple NestJS GraphQL resolver with a query returning a greeting and a mutation updating a message.

Execution Table

Step	Request Type	Resolver Method	Action	Result
1	Query	hello	Called hello()	Returns 'Hello World'
2	Mutation	updateMessage	Called updateMessage with 'Hi!'	Returns 'Hi!'
3	Query	hello	Called hello()	Returns 'Hello World'
4	Mutation	updateMessage	Called updateMessage with 'Bye!'	Returns 'Bye!'
5	End	-	No more requests	Execution stops

💡 No more client requests; server waits for next GraphQL operation.

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
newMsg	undefined	'Hi!'	'Bye!'	'Bye!'

Key Moments - 2 Insights

Why does the query 'hello' always return the same result?

Because the 'hello' query method returns a fixed string without changing any state, as shown in execution_table rows 1 and 3.

How does the mutation 'updateMessage' change the output?

The mutation takes an argument 'newMsg' and returns it, changing the output each time it's called, as seen in execution_table rows 2 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what does the 'hello' query return at step 3?

A 'Hello World'

B 'Hi!'

C 'Bye!'

D undefined

💡 Hint

Check the 'Result' column for step 3 in the execution_table.

At which step does the mutation receive the argument 'Hi!'?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column for when 'updateMessage' is called with 'Hi!' in the execution_table.

If the mutation 'updateMessage' always returned 'Hello', how would the variable 'newMsg' change?

A It would be undefined

B It would alternate between 'Hi!' and 'Bye!'

C It would always be 'Hello'

D It would cause an error

💡 Hint

Refer to variable_tracker to see how 'newMsg' changes with mutation calls.

Concept Snapshot

NestJS GraphQL uses resolvers with @Query for fetching data and @Mutation for changing data.
Queries do not change state; mutations can accept arguments to update data.
Each resolver method returns data sent back to the client.
Use @Args to get input values for mutations.
Queries and mutations are separate methods but both return responses.

Full Transcript

In NestJS GraphQL, clients send requests that are either queries or mutations. Queries fetch data without changing anything, while mutations modify data. The resolver class uses decorators @Query and @Mutation to mark methods for these operations. For example, a 'hello' query returns a fixed greeting string. A mutation called 'updateMessage' takes an argument and returns it, simulating data change. The execution table shows step-by-step calls and results. Variables like 'newMsg' track mutation inputs. Understanding this flow helps beginners see how GraphQL operations work in NestJS.