Spring Data Jpa

@Entity
@Table(name = "orders", indexes = {
    @Index(name = "idx_orders_customer_id", columnList = "customer_id"),
    @Index(name = "idx_orders_status_created", columnList = "status, created_at")
})
@Getter
@NoArgsConstructor(access = AccessLevel.PROTECTED)
public class Order {

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

    @Version
    private Long version;

    @Column(name = "customer_id", nullable = false, updatable = false)
    private UUID customerId;

    @Enumerated(EnumType.STRING)
    @Column(nullable = false, length = 32)
    private OrderStatus status;

    @Embedded
    private Money total;

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

    @CreationTimestamp
    @Column(name = "created_at", nullable = false, updatable = false)
    private Instant createdAt;

    @UpdateTimestamp
    @Column(name = "updated_at", nullable = false)
    private Instant updatedAt;

    public static Order create(UUID customerId) {
        Order order = new Order();
        order.customerId = Objects.requireNonNull(customerId);
        order.status = OrderStatus.DRAFT;
        order.total = Money.zero("EUR");
        return order;
    }

    public void addItem(UUID productId, int quantity, Money unitPrice) {
        if (status != OrderStatus.DRAFT) {
            throw new IllegalStateException("Cannot edit submitted order");
        }
        items.add(OrderItem.create(this, productId, quantity, unitPrice));
        recalculateTotal();
    }

    private void recalculateTotal() {
        total = items.stream()
            .map(OrderItem::subtotal)
            .reduce(Money.zero("EUR"), Money::add);
    }
}

@Embeddable
public record Money(
    @Column(name = "amount", nullable = false, precision = 19, scale = 2)
    BigDecimal amount,

    @Column(name = "currency", nullable = false, length = 3)
    String currency
) {
    public Money {
        Objects.requireNonNull(amount);
        Objects.requireNonNull(currency);
        if (amount.signum() < 0) {
            throw new IllegalArgumentException("Amount cannot be negative");
        }
    }

    public static Money zero(String currency) {
        return new Money(BigDecimal.ZERO, currency);
    }

    public Money add(Money other) {
        if (!currency.equals(other.currency)) {
            throw new IllegalArgumentException("Currency mismatch");
        }
        return new Money(amount.add(other.amount), currency);
    }

    public Money multiply(int quantity) {
        if (quantity < 1) {
            throw new IllegalArgumentException("Quantity must be positive");
        }
        return new Money(amount.multiply(BigDecimal.valueOf(quantity)), currency);
    }
}

public record OrderResponse(UUID id, String status, BigDecimal total, Instant createdAt) {
    static OrderResponse from(Order order) {
        return new OrderResponse(
            order.getId(),
            order.getStatus().name(),
            order.getTotal().amount(),
            order.getCreatedAt());
    }
}

@Entity
@Getter
@NoArgsConstructor(access = AccessLevel.PROTECTED)
class OrderItem {

    @Id
    @GeneratedValue(strategy = GenerationType.UUID)
    private UUID id;

    @ManyToOne(fetch = FetchType.LAZY, optional = false)
    @JoinColumn(name = "order_id", nullable = false, foreignKey = @ForeignKey(name = "fk_order_item_order"))
    private Order order;

    @Column(name = "product_id", nullable = false, updatable = false)
    private UUID productId;

    private int quantity;
    private Money unitPrice;

    static OrderItem create(Order order, UUID productId, int quantity, Money unitPrice) {
        OrderItem item = new OrderItem();
        item.order = Objects.requireNonNull(order);
        item.productId = Objects.requireNonNull(productId);
        item.quantity = quantity;
        item.unitPrice = Objects.requireNonNull(unitPrice);
        return item;
    }

    Money subtotal() {
        return unitPrice.multiply(quantity);
    }
}

@Override
public boolean equals(Object other) {
    return other instanceof Customer that
        && email != null
        && email.equals(that.getEmail());
}

@Override
public int hashCode() {
    return email == null ? 0 : email.hashCode();
}

public interface OrderRepository extends JpaRepository<Order, UUID> {

    boolean existsByCustomerIdAndStatus(UUID customerId, OrderStatus status);

    Optional<Order> findByIdAndCustomerId(UUID id, UUID customerId);

    @EntityGraph(attributePaths = {"items"})
    Optional<Order> findById(UUID id);

    @Query("""
        select o
        from Order o
        where o.status = :status
        order by o.createdAt desc, o.id desc
        """)
    List<Order> findRecentByStatus(OrderStatus status, Limit limit);
}

@EntityGraph(attributePaths = {"items", "items.product"})
Optional<Order> findWithItemsAndProductsById(UUID id);

public interface OrderSummary {
    UUID getId();
    UUID getCustomerId();
    OrderStatus getStatus();
    Instant getCreatedAt();
}

List<OrderSummary> findByStatus(OrderStatus status);