data-repository-pattern.md

---

title: Isolate Database Access Behind Repositories impact: CRITICAL impactDescription: Enables technology changes without codebase-wide refactors tags: data, repository, prisma, nestjs, isolation

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Isolate Database Access Behind Repositories

Impact: CRITICAL

All database access goes through Repository classes. Repositories are the only code that knows about Prisma. Services depend on repositories via NestJS Dependency Injection.

Incorrect (Prisma leaking into service):

@Injectable()
export class ItemService {
  constructor(private prisma: PrismaService) {}

  async findBySlug(slug: string) {
    return this.prisma.item.findFirst({
      where: { slug },
      include: { seller: true }
    });
  }
}

Correct (Repository abstraction):

// repositories/item.repository.ts
@Injectable()
export class ItemRepository {
  constructor(private readonly prisma: PrismaService) {}

  async findBySlug(slug: string): Promise<ItemDetailDTO | null> {
    const item = await this.prisma.$replica().item.findFirst({
      where: { slug, status: 'PUBLISHED' },
      select: {
        id: true,
        title: true,
        slug: true,
        price: true,
        seller: {
          select: { id: true, name: true, logoUrl: true }
        }
      }
    });
    return item;
  }
}

// item.service.ts — no Prisma knowledge
@Injectable()
export class ItemService {
  constructor(private readonly itemRepo: ItemRepository) {}

  async findBySlug(slug: string): Promise<ItemDetailDTO | null> {
    return this.itemRepo.findBySlug(slug);
  }
}

Rules:

• All Prisma calls live in *Repository classes
• Repositories return DTOs or plain objects, never raw Prisma types
• No business logic in repositories — only data access
• Services depend on repositories, never on PrismaService directly
• Repositories use $replica() for read operations, default client for writes