Repository pattern — don't expose the ORM to services

Tomorrow you swap TypeORM for Prisma or vice versa — change the repository only, services don't move.

NestJS·NestJS Data & Errors·Medium·Since 2026-05-01

nestjs-data-repository-pattern

Why it matters

Failure modes if this rule is ignored

Stake	If ignored
Hard to change	ORM swap forces edits across every service — persistence leaks past the repository boundary.
Hard to test	In tests, you can mock SitesRepository without mocking the entire ORM.
Layer mixing	ORMs leak abstractions (lazy loading, transactions, eager fetching). The repository layer seals the leak.

How to fix

A service does not receive Repository<Site> from TypeORM or PrismaClient. It receives a SitesRepository you wrote — a clear interface.

Examples

Bad

@Injectable()
export class SitesService {
  constructor(@InjectRepository(Site) private repo: Repository<Site>) {}

  async getSite(id: string) {
    return this.repo
      .createQueryBuilder('site')
      .leftJoinAndSelect('site.owner', 'owner')
      .where('site.id = :id', { id })
      .getOne();
    // service knows about TypeORM QueryBuilder
  }
}

Good

// libs/sites-management/backend-data-access/src/lib/sites.repository.ts
export interface SitesRepository {
  findById(id: string): Promise<Site | null>;
  findByOwner(ownerId: string): Promise<Site[]>;
  create(input: CreateSiteInput): Promise<Site>;
}

@Injectable()
export class TypeOrmSitesRepository implements SitesRepository {
  constructor(@InjectRepository(SiteEntity) private repo: Repository<SiteEntity>) {}

  async findById(id: string): Promise<Site | null> {
    const entity = await this.repo.findOne({ where: { id }, relations: ['owner'] });
    return entity ? toDomain(entity) : null;
  }
}

// libs/sites-management/backend-feature/src/lib/sites.service.ts
@Injectable()
export class SitesService {
  constructor(@Inject(SITES_REPOSITORY) private repo: SitesRepository) {}

  async getSite(id: string) {
    return this.repo.findById(id);
  }
}

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