Adding unique album validator

backend/src/album/album.entity.ts

@@ -1,5 +1,6 @@
 import { Entity, Column, OneToMany, PrimaryGeneratedColumn } from 'typeorm';
 import { IsNotEmpty, IsString } from 'class-validator';
+import { AlbumTitleUnique } from './middleware/uniqueAlbumTitle.middleware';
 import { Song } from '../song/song.entity';
 
 @Entity()

backend/src/album/middleware/uniqueAlbumTitle.middleware.ts

@@ -0,0 +1,32 @@
+import { registerDecorator, ValidationOptions, ValidatorConstraint, ValidatorConstraintInterface } from 'class-validator';
+import { Injectable } from '@nestjs/common';
+import { InjectRepository } from '@nestjs/typeorm';
+import { Repository } from 'typeorm';
+
+@ValidatorConstraint({ async: true })
+export class UniqueAlbumTitle implements ValidatorConstraintInterface {
+  constructor(@InjectRepository(Album)
+  private albumRepository: Repository<Album>
+) {}
+
+  validate(albumTitle: string) {
+    return this.albumRepository.findOneBy({
+      where: {
+        id: title
+      }).then((albumTitle) => {
+      return albumTitle === undefined;
+    });
+  }
+}
+
+export function AlbumTitleUnique(validationOptions?: ValidationOptions) {
+  return function (object: object, propertyName: string) {
+    registerDecorator({
+      target: object.constructor,
+      propertyName: propertyName,
+      options: validationOptions,
+      constraints: [],
+      validator: AlbumTitleUnique,
+    });
+  };
+}