You have entities and repositories set up. Now let’s work through every data operation in depth — create, read, update, delete — and the JPA mechanics behind each. Create: save() @Service @RequiredArgsConstructor @Transactional public class OrderService { private final OrderRepository orderRepository; public Order createOrder(CreateOrderRequest request) { Order order = new Order(); order.setCustomerId(request.customerId()); order.setOrderNumber(generateOrderNumber()); order.setStatus(OrderStatus.PENDING); request.items().forEach(itemReq -> { OrderItem item = new OrderItem(); item.setProductId(itemReq.productId()); item.setQuantity(itemReq.quantity()); item.setUnitPrice(itemReq.unitPrice()); order.addItem(item); // manages bidirectional relationship }); return orderRepository.