How do you write a custom pipe for parsing and validating a MongoDB ObjectId param in NestJS?

Q: How do you write a custom pipe for parsing and validating a MongoDB ObjectId param in NestJS?

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common'; import { Types } from 'mongoose'; @Injectable() export class ParseObjectIdPipe implements PipeTransform { transform(value: string): Types.ObjectId { if (!Types.ObjectId.isValid(value)) { throw new BadRequestException(`"${value}" is not a valid ObjectId`); } return new Types.ObjectId(value); } } // Usage in controller @Get(':id') findOne(@Param('id', ParseObjectIdPipe) id: Types.ObjectId) { return this.service.findById(id); // typed as ObjectId, not string } @Delete(':id') remove(@Param('id', ParseObjectIdPipe) id: Types.ObjectId) { return this.service.delete(id); } PipeTransform generics make the transformation contract explicit. Always throw BadRequestException (400) for invalid input — not NotFoundException. Return the transformed type — handlers should receive the final type, not raw strings. Pipes are @Injectable() — they can inject services for async database-backed validation. For async validation, return Promise from transform().

Difficulty: 6/10

NestJS pipes, ObjectId validation, MongoDB integration

Implement PipeTransform with typed generics, validate the value using Types.ObjectId.isValid(), throw BadRequestException if invalid, and return a Types.ObjectId instance if valid. The pipe both validates and transforms — the handler receives a strongly typed ObjectId, not a raw string.

ParseObjectIdPipe implementation and usage

Custom pipe design principles:

1
PipeTransform<Input, Output> generics make the transformation contract explicit.
2
Always throw BadRequestException (400) for invalid input — not NotFoundException.
3
Return the transformed type — handlers should receive the final type, not raw strings.
4
Pipes are @Injectable() — they can inject services for async database-backed validation.
5
For async validation, return Promise<Output> from transform().

Scenario Questions

0-2 years experience

1We have a GET endpoint `/users/:id`. How would you implement a custom pipe to ensure the `id` param is a valid MongoDB ObjectId before it reaches the controller?
2If you forget to call `next.handle()` inside your pipe, what will happen to the request processing?
3What HTTP response does NestJS send when your pipe throws a BadRequestException for an invalid ObjectId?

Follow-up Questions

What edge cases did you think about when designing the pipe?
How would you unit‑test this validation logic?
If validation needed to check database existence, how would you extend the pipe?