Spring Data JPA

Entity Conventions

@Entity
@Table(name = "orders")
@Getter
@NoArgsConstructor(access = AccessLevel.PROTECTED) // JPA requires no-arg, hide from callers
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    @Column(updatable = false, nullable = false)
    private UUID id;

    @Column(nullable = false)
    private String customerEmail;

    @Enumerated(EnumType.STRING) // always STRING, never ORDINAL
    @Column(nullable = false)
    private OrderStatus status;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<OrderItem> items = new ArrayList<>();

    @CreationTimestamp
    @Column(updatable = false)
    private Instant createdAt;

    @UpdateTimestamp
    private Instant updatedAt;

    // Static factory, not public constructor
    public static Order create(String customerEmail) {
        Order order = new Order();
        order.customerEmail = customerEmail;
        order.status = OrderStatus.PENDING;
        return order;
    }

    // Behavior on entity, not in service
    public void addItem(Product product, int quantity) {
        items.add(OrderItem.create(this, product, quantity));
    }
}

Rules

• @Enumerated(EnumType.STRING) always — ORDINAL breaks on enum reordering
• GenerationType.UUID for IDs — never expose auto-increment integers
• @NoArgsConstructor(access = PROTECTED) — required by JPA, hidden from app code
• @Getter from Lombok — no @Setter on entities (use behavior methods)
• Collections initialized inline (= new ArrayList<>()) — never null

N+1 Prevention

Identify: One query for orders + N queries for each order's items = N+1.

Fix with JOIN FETCH:

@Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.id = :id")
Optional<Order> findByIdWithItems(@Param("id") UUID id);

// For lists — use @EntityGraph to avoid duplicates
@EntityGraph(attributePaths = {"items", "items.product"})
List<Order> findByStatus(OrderStatus status);

Fix with Projections for read-only views:

// Interface projection — no entity loaded
public interface OrderSummary {
    UUID getId();
    String getCustomerEmail();
    OrderStatus getStatus();
    Instant getCreatedAt();
}

List<OrderSummary> findByStatus(OrderStatus status); // fast, no lazy loading issues

Query Patterns

public interface OrderRepository extends JpaRepository<Order, UUID> {

    // Derived query — simple conditions
    List<Order> findByStatusAndCustomerEmail(OrderStatus status, String email);

    // JPQL — for joins and complex conditions
    @Query("SELECT o FROM Order o JOIN FETCH o.items WHERE o.status = :status")
    List<Order> findActiveOrdersWithItems(@Param("status") OrderStatus status);

    // Native SQL — only when JPQL can't do it
    @Query(value = "SELECT * FROM orders WHERE created_at > NOW() - INTERVAL '7 days'",
           nativeQuery = true)
    List<Order> findRecentOrders();

    // Exists check — faster than findById + isPresent
    boolean existsByCustomerEmailAndStatus(String email, OrderStatus status);

    // Projection
    List<OrderSummary> findByCustomerEmail(String email);
}

Pagination

// Always use Pageable for list endpoints
Page<Order> findByStatus(OrderStatus status, Pageable pageable);

// In service
Page<Order> orders = orderRepository.findByStatus(status, PageRequest.of(page, size, Sort.by("createdAt").descending()));

Bidirectional Relationships

// Parent side (Order)
@OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
private List<OrderItem> items = new ArrayList<>();

// Child side (OrderItem) — owns the FK
@ManyToOne(fetch = FetchType.LAZY) // LAZY always on @ManyToOne
@JoinColumn(name = "order_id", nullable = false)
private Order order;

// Helper on parent to keep both sides in sync
public void addItem(OrderItem item) {
    items.add(item);
    item.setOrder(this);
}

Gotchas

• Agent uses FetchType.EAGER — always use LAZY on @ManyToOne and @ManyToMany
• Agent uses @Enumerated(EnumType.ORDINAL) — always use STRING
• Agent uses Long IDs — use UUID
• Agent calls findAll() for list endpoints — always use Pageable
• Agent adds setters to entities — use behavior methods instead
• Agent forgets orphanRemoval = true on @OneToMany — child records become orphans
• Agent writes N+1 without realizing — check for items access in loops