jpa-patterns

# JPA/Hibernate Patterns

Use for data modeling, repositories, and performance tuning in Spring Boot.

## Entity Design

```java
@Entity
@Table(name = "markets", indexes = {
  @Index(name = "idx_markets_slug", columnList = "slug", unique = true)
})
@EntityListeners(AuditingEntityListener.class)
public class MarketEntity {
  @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  @Column(nullable = false, length = 200)
  private String name;

  @Column(nullable = false, unique = true, length = 120)
  private String slug;

  @Enumerated(EnumType.STRING)
  private MarketStatus status = MarketStatus.ACTIVE;

  @CreatedDate private Instant createdAt;
  @LastModifiedDate private Instant updatedAt;
}
```

Enable auditing:
```java
@Configuration
@EnableJpaAuditing
class JpaConfig {}
```

## Relationships and N+1 Prevention

```java
@OneToMany(mappedBy = "market", cascade = CascadeType.ALL, orphanRemoval = true)
private List<PositionEntity> positions = new ArrayList<>();
```

- Default to lazy loading; use `JOIN FETCH` in queries when needed
- Avoid `EAGER` on collections; use DTO projections for read paths

```java
@Query("select m from MarketEntity m left join fetch m.positions where m.id = :id")
Optional<MarketEntity> findWithPositions(@Param("id") Long id);
```

## Repository Patterns

```java
public interface MarketRepository extends JpaRepository<MarketEntity, Long> {
  Optional<MarketEntity> findBySlug(String slug);

  @Query("select m from MarketEntity m where m.status = :status")
  Page<MarketEntity> findByStatus(@Param("status") MarketStatus status, Pageable pageable);
}
```

- Use projections for lightweight queries:
```java
public interface MarketSummary {
  Long getId();
  String getName();
  MarketStatus getStatus();
}
Page<MarketSummary> findAllBy(Pageable pageable);
```

## Transactions

- Annotate service methods with `@Transactional`
- Use `@Transactional(readOnly = true)` for read paths to optimize
- Choose propagation carefully; avoi