如何使用 JPA 存储库将涉及来自多个表的列的本机查询结果的结果 map 转换为自定义 class？

Question

I did try to find some similar questions but I could not understand any for my problem. I have a Repository as follows

@Repository
public interface CartRepository extends JpaRepository<Cart, Long> {

    @Query(value = "Select i.item_desc, i.item_name ,sum(price) as price, count(*) as quantity from items  i, cart_items ci, cart c where ci.items_item_id=i.item_id and c.id = ci.cart_id and c.user_id = :userId group by ci.items_item_id", nativeQuery = true)
    public List<Object> getCartItemsForCustomer(long userId);

}

The Query result will have four fields, item_desc, item_name, price and quantity.

I have made a DTO class to return the results as a List of DTO object.

public class CartItemDto {

   private String itemName;
   private String itemDesc;
   private Long price;
   private Integer quanity;
}

I am not understanding how to Map the result into the custom dto class.

List<Object> objs = cartRepository.getCartItemsForCustomer(userId);

Please suggest the way to convert the list of objects to List of custom class.

Answer 1

You can solve this using Interface-based Projections like this:

@Query(value = "Select i.item_desc as itemDesc, i.item_name as itemName, sum(price) as price, count(*) as quantity from items  i, cart_items ci, cart c where ci.items_item_id=i.item_id and c.id = ci.cart_id and c.user_id = ?1 group by ci.items_item_id", nativeQuery = true)
public List<CartItemInterface> getCartItemsForCustomer(long userId);

CartItemInterface

public interface CartItemInterface {

    String getItemDesc();
    String getItemName();
    Long getPrice();
    Integer getQuantity();
}

Answer 2

You can use DTO projection and map columns to a Java Object.

• You need to create a constructor which accepts these four fields and creates the object.
• change the select query to give fully qualified name of the class and call its constructor.

Note: If you are using static class then reference using $ instead of (.)

public class CartItemDto {
   private String itemName;
   private String itemDesc;
   private Long price;
   private Integer quanity;
   public CartItemDto(String itemName, String itemDesc, Long price, Integer quanity) {
      this.itemName = itemName;
      this.itemDesc = itemDesc;
      this.price = price;
      this.quantity = quantity;
   }   

}

Now update the query to instantiate the dto object for each row in the result set and return list of CartItemDto objects.

@Query(value = "Select new com.package.CartItemDto(i.item_desc, i.item_name ,sum(price), count(*)) from items  i, cart_items ci, cart c where ci.items_item_id=i.item_id and c.id = ci.cart_id and c.user_id = :userId group by ci.items_item_id")
    public List<CartItemDto> getCartItemsForCustomer(long userId);

Note: This won't work with native queries. you need to update the query and change column and table names to field and entity names. Also change nativeQuery flag to false (which is by default false so just remove it)