Projection of native query results to DTO

Question

I want to map results to data transfer objects but I don't know what is the proper way for this issue.

Since there are lots of result rows I don't think it's a nice solution if I parse the return objects for each of them and assign them to currently created data transfer object.

public class MaintenanceTaskRepositoryImpl implements CustomMaintenanceTaskRepository {

@Autowired
EntityManager em;

@Override
public List<Object []> findRepairCostForLast5Year() {


    List<Object []> results = em.createNativeQuery(
            "SELECT              YEAR(END_DATE)      AS YEAR," +
                    "                        SUM(PRICE)          AS REPAIR_COST" +
                    "    FROM                MAINTENANCE_TASK" +
                    "    WHERE               TYPE_OF_WORK = 'PREVENTIVE' AND" +
                    "                        DATEDIFF(YEAR, END_DATE , CURRENT_DATE) < 6" +
                    "    GROUP BY        YEAR(END_DATE) " +
                    "    LIMIT 5").getResultList();



    return results;



}}

Above query returns multiple rows that's why it's a list of object array. It works right now but I want my method directly returns list of DTO.

My DTO class is below.

public class RepairCostDto {
private int year;
private BigDecimal cost;

public RepairCostDto(int year, BigDecimal cost) {
    this.year = year;
    this.cost = cost;
}

public RepairCostDto() {
}

public int getYear() {
    return year;
}

public void setYear(int year) {
    this.year = year;
}

public BigDecimal getCost() {
    return cost;
}

public void setCost(BigDecimal cost) {
    this.cost = cost;
}}

Thank you in advance.

Edit:

above return is like,

[[year,cost],[year1,cost1],[year2,cost2]]

but I want it as below,

[RepairCostObj,RepairCostObj1, RepairCostObj2...]

Edit2:

Referring this and this answers should I apply the solution that I mentioned above?

Answer 1

this should be what you want

@PersistenceContext
private EntityManager entityManager;


@Transactional
public List<RepairCostDto> listRepairCost(){
    String sql = "xxxxxxxx";

    SQLQuery sqlQuery = entityManager.createNativeQuery(sql).unwrap(SQLQuery.class);
    Query query = sqlQuery.setResultTransformer(Transformers.aliasToBean(RepairCostDto.class));
    List<RepairCostDto> list = query.list();
    entityManager.clear();
    return list;
}

Also, I think specification would be a better way to do it.