Validates if value exists or is unique in database. Using class-validator and typorm.

IsExisting.ts

import {getRepository} from 'typeorm';
import {ValidationArguments, ValidatorConstraint, ValidatorConstraintInterface} from "class-validator";

@ValidatorConstraint({name: "exists", async: true})
export class IsExisting implements ValidatorConstraintInterface {

    async validate(value: any, args: ValidationArguments) {
        return 0 !== await getRepository(args.constraints[0]).count({
            where: {
                [args.constraints[1]]: value
            }
        });
    }

    defaultMessage(args: ValidationArguments) {
        return `The selected ${args.property} is invalid.`;
    }

}

IsUnique.ts

import {getRepository} from 'typeorm';
import {ValidationArguments, ValidatorConstraint, ValidatorConstraintInterface} from "class-validator";

@ValidatorConstraint({name: "unique", async: true})
export class IsUnique implements ValidatorConstraintInterface {

    async validate(value: any, args: ValidationArguments) {
        return 0 === await getRepository(args.constraints[0]).count({
            where: {
                [args.constraints[1]]: value
            }
        });
    }

    defaultMessage(args: ValidationArguments) {
        return `The ${args.property} has already been taken.`;
    }

}

Usage:

import {Validate} from 'class-validator';
import {IsUnique} from "../Constraints/IsUnique";
import {User} from "../Entities/User";

export class Registration {
    @Validate(IsUnique, [User, 'email'])
    email: string;
}