1---
2title: Use Repository Pattern for Data Access
3impact: HIGH
4impactDescription: Decouples business logic from database
5tags: architecture, repository, data-access
6---
7 
8## Use Repository Pattern for Data Access
9 
10Create custom repositories to encapsulate complex queries and database logic. This keeps services focused on business logic, makes testing easier with mock repositories, and allows changing database implementations without affecting business code.
11 
12**Incorrect (complex queries in services):**
13 
14```typescript
15// Complex queries in services
16@Injectable()
17export class UsersService {
18  constructor(
19    @InjectRepository(User) private repo: Repository<User>,
20  ) {}
21 
22  async findActiveWithOrders(minOrders: number): Promise<User[]> {
23    // Complex query logic mixed with business logic
24    return this.repo
25      .createQueryBuilder('user')
26      .leftJoinAndSelect('user.orders', 'order')
27      .where('user.isActive = :active', { active: true })
28      .andWhere('user.deletedAt IS NULL')
29      .groupBy('user.id')
30      .having('COUNT(order.id) >= :min', { min: minOrders })
31      .orderBy('user.createdAt', 'DESC')
32      .getMany();
33  }
34 
35  // Service becomes bloated with query logic
36}
37```
38 
39**Correct (custom repository with encapsulated queries):**
40 
41```typescript
42// Custom repository with encapsulated queries
43@Injectable()
44export class UsersRepository {
45  constructor(
46    @InjectRepository(User) private repo: Repository<User>,
47  ) {}
48 
49  async findById(id: string): Promise<User | null> {
50    return this.repo.findOne({ where: { id } });
51  }
52 
53  async findByEmail(email: string): Promise<User | null> {
54    return this.repo.findOne({ where: { email } });
55  }
56 
57  async findActiveWithMinOrders(minOrders: number): Promise<User[]> {
58    return this.repo
59      .createQueryBuilder('user')
60      .leftJoinAndSelect('user.orders', 'order')
61      .where('user.isActive = :active', { active: true })
62      .andWhere('user.deletedAt IS NULL')
63      .groupBy('user.id')
64      .having('COUNT(order.id) >= :min', { min: minOrders })
65      .orderBy('user.createdAt', 'DESC')
66      .getMany();
67  }
68 
69  async save(user: User): Promise<User> {
70    return this.repo.save(user);
71  }
72}
73 
74// Clean service with business logic only
75@Injectable()
76export class UsersService {
77  constructor(private usersRepo: UsersRepository) {}
78 
79  async getActiveUsersWithOrders(): Promise<User[]> {
80    return this.usersRepo.findActiveWithMinOrders(1);
81  }
82 
83  async create(dto: CreateUserDto): Promise<User> {
84    const existing = await this.usersRepo.findByEmail(dto.email);
85    if (existing) {
86      throw new ConflictException('Email already registered');
87    }
88 
89    const user = new User();
90    user.email = dto.email;
91    user.name = dto.name;
92    return this.usersRepo.save(user);
93  }
94}
95```
96 
97Reference: [Repository Pattern](https://martinfowler.com/eaaCatalog/repository.html)
98