jpashop : 컬렉션 조회 v5 (jpa - dto 직접조회 최적화)

jpashop/src/main/java/com/example/jpashop/api/OrderApiController.java

@@ -63,4 +63,9 @@ public class OrderApiController {
     public List<OrderQueryDto> ordersV4() {
         return orderQueryRepository.findOrderQueryDtos();
     }
+
+    @GetMapping("/api/v5/orders")
+    public List<OrderQueryDto> ordersV5() {
+        return orderQueryRepository.findAllByDto_optimization();
+    }
 }

jpashop/src/main/java/com/example/jpashop/repository/order/query/OrderQueryRepository.java

@@ -5,6 +5,8 @@ import org.springframework.stereotype.Repository;
 
 import javax.persistence.EntityManager;
 import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
 
 @Repository
 @RequiredArgsConstructor
@@ -22,6 +24,36 @@ public class OrderQueryRepository {
         return result;
     }
 
+    public List<OrderQueryDto> findAllByDto_optimization() {
+        List<OrderQueryDto> result = findOrders();
+        List<Long> orderIds = toOrderIds(result);
+
+        Map<Long, List<OrderItemQueryDto>> orderItemMap = findOrderItemMap(orderIds);
+
+        result.forEach(o -> o.setOrderItems(orderItemMap.get(o.getOrderId())));
+        return result;
+    }
+
+    private Map<Long, List<OrderItemQueryDto>> findOrderItemMap(List<Long> orderIds) {
+        List<OrderItemQueryDto> orderItems = em.createQuery(
+                "select new com.example.jpashop.repository.order.query.OrderItemQueryDto(oi.order.id, i.name, oi.orderPrice, oi.count)" +
+                        " from OrderItem oi" +
+                        " join oi.item i" +
+                        " where oi.order.id in :orderIds", OrderItemQueryDto.class)
+                .setParameter("orderIds", orderIds)
+                .getResultList();
+
+        Map<Long, List<OrderItemQueryDto>> orderItemMap = orderItems.stream()
+                .collect(Collectors.groupingBy(OrderItemQueryDto::getOrderId));
+        return orderItemMap;
+    }
+
+    private List<Long> toOrderIds(List<OrderQueryDto> result) {
+        return result.stream()
+                .map(OrderQueryDto::getOrderId)
+                .collect(Collectors.toList());
+    }
+
     private List<OrderItemQueryDto> findOrderItems(Long orderId) {
         return em.createQuery(
                 "select new com.example.jpashop.repository.order.query.OrderItemQueryDto(oi.order.id, i.name, oi.orderPrice, oi.count)" +