장바구니에 담긴 아이템을 삭제하고 싶습니다.

장바구니에 담긴 아이템을 데이터베이스에서 삭제하는 기능을 추가하려고 합니다.

controller

//주문한 아이템 식제
@DeleteMapping("/orders/{orderId}/orderList")
public String deleteOrderItem(@PathVariable("orderId") Long orderId){
    orderItemService.deleteOrderItem(orderId);
    return "orders/orderForm";
}

repository

@Repository
@RequiredArgsConstructor
public class OrderItemRepository {
    private final EntityManager em;
    public void save(OrderItem orderItem) {
        em.persist(orderItem);
    }
    public OrderItem findOne(Long id) {
        return em.find(OrderItem.class, id);
    }

    public List<OrderItem> findAll() {
        return em.createQuery("select i from OrderItem i",OrderItem.class).getResultList();
    }

    //해당 orderId의 OrderItem을 검색
    public  List<OrderItem> findOrder(Long orderId) {
        String query = "select i from OrderItem i where i.order = "+orderId;
        return em.createQuery(query,OrderItem.class).getResultList();
    }

    //해당 OrderId의 orderItem 삭제
    public void deleteOrderItem(Long orderId) {
        em.remove(findOrder(orderId));
    }
}

service

@Service
@Transactional(readOnly = true)
@RequiredArgsConstructor
public class OrderItemService {

    private final OrderRepository orderRepository;
    private final OrderItemRepository orderItemRepository;
    private final ItemRepository itemRepository;


    /**
     * 주문
     */
    @Transactional
    public Long orderItem(Long orderId,Long itemId, String temperature, String size, int amount, int sizeUp) {

        //엔티티 조회
        Order order = orderRepository.findOne(orderId);
        Item item = itemRepository.findOne(itemId);

        //주문상품 생성
        OrderItem orderItem = OrderItem.createOrderItem(order ,item, item.getName() ,item.getPrice(), temperature, size, amount, sizeUp);


        //주문 저장
        orderItemRepository.save(orderItem);
        return orderItem.getOrder().getOrderId();
    }

    public List<OrderItem> findOrderItems() {
        return orderItemRepository.findAll();
    }


    //주문아이템 삭제
    public void deleteOrderItem(Long orderId){
        orderItemRepository.deleteOrderItem(orderId);
    }

findOrder 메소드로 해당하는 주문아이템을 찾아서

findOrder(orderId)

entitiymanager의 remove메소드를 이용해서  삭제를 시도해봤습니다.

em.remove(findOrder(orderId));

하지만 삭제 버튼을 누르면

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: Unknown entity: java.util.ArrayList; nested exception is java.lang.IllegalArgumentException: Unknown entity: java.util.ArrayList] with root cause

org.hibernate.MappingException: Unknown entity: java.util.ArrayList

해당 오류가 뜹니다. 

도움주시면 감사하겠습니다ㅠ