How do you write an async custom validator that checks uniqueness against a database in NestJS?

Q: How do you write an async custom validator that checks uniqueness against a database in NestJS?

// main.ts — allow class-validator to use the NestJS container import { useContainer } from 'class-validator'; const app = await NestFactory.create(AppModule); useContainer(app.select(AppModule), { fallbackOnErrors: true }); // Constraint with injected service @ValidatorConstraint({ name: 'isEmailUnique', async: true }) @Injectable() export class IsEmailUniqueConstraint implements ValidatorConstraintInterface { constructor(private usersService: UsersService) {} async validate(email: string): Promise { const user = await this.usersService.findByEmail(email); return !user; // true = valid (email not taken) } defaultMessage(): string { return 'Email is already in use'; } } // Register as a NestJS provider in the relevant module // providers: [IsEmailUniqueConstraint] // Convenience decorator export function IsEmailUnique() { return (object: object, propertyName: string) => registerDecorator({ target: object.constructor, propertyName, validator: IsEmailUniqueConstraint, async: true }); } export class RegisterDto { @IsEmail() @IsEmailUnique() email: string; } Call useContainer(app.select(AppModule), { fallbackOnErrors: true }) in main.ts before app.listen(). Decorate the constraint class with @Injectable() so NestJS manages its instantiation. Register the constraint class in the providers array of the relevant module. Set async: true on both @ValidatorConstraint and in registerDecorator options. Without useContainer(), the injected service is always undefined — a silent failure.

Difficulty: 6/10

custom validation, async validators, database integration

Set async: true on the constraint, decorate it with @Injectable() so NestJS manages it, and call useContainer() in main.ts to allow class-validator to resolve services from the NestJS DI container. Register the constraint class as a provider in your module.

Async uniqueness validator with DI

Required setup steps for async validators with DI:

1
Call useContainer(app.select(AppModule), { fallbackOnErrors: true }) in main.ts before app.listen().
2

How do you write an async custom validator that checks uniqueness against a database in NestJS?

Difficulty: 6/10

custom validation, async validators, database integration

Set async: true on the constraint, decorate it with @Injectable() so NestJS manages it, and call useContainer() in main.ts to allow class-validator to resolve services from the NestJS DI container. Register the constraint class as a provider in your module.

Async uniqueness validator with DI

Required setup steps for async validators with DI:

1
Call useContainer(app.select(AppModule), { fallbackOnErrors: true }) in main.ts before app.listen().
2

How do you write an async custom validator that checks uniqueness against a database in NestJS?

Set async: true on the constraint, decorate it with @Injectable() so NestJS manages it, and call useContainer() in main.ts to allow class-validator to resolve services from the NestJS DI container. Register the constraint class as a provider in your module.

How do you write an async custom validator that checks uniqueness against a database in NestJS?

Set async: true on the constraint, decorate it with @Injectable() so NestJS manages it, and call useContainer() in main.ts to allow class-validator to resolve services from the NestJS DI container. Register the constraint class as a provider in your module.

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