从查询中获取结果作为自定义对象列表答案

【问题标题】：Get result from query as custom List of Objects从查询中获取结果作为自定义对象列表
【发布时间】：2018-11-06 20:23:03
【问题描述】：

我想为显示过去 10 天每天的交易量的条形图实现 SQL 查询。例如我有这个表结构：

CREATE TABLE `payment_transactions` (
  `id` int(11) NOT NULL,
  `amount` int(11) DEFAULT NULL,
  `created_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
);

我得到了这个示例结果（每天分组）：

Date       | Amount| Number of transactions per day |
11-11-2018 | 30    | 3                              |
11-12-2018 | 230   | 13                             |

JPA 查询：

public List<DashboardDTO> findAll() {

        String hql = "SELECT date(created_at) AS cdate, sum(amount) AS amount, count(id) AS nooftransaction "
                + "FROM payment_transactions WHERE date(created_at)>=date(now()- interval 10 DAY) "
                + "AND date(created_at)<date(now()) GROUP BY date(created_at)";

        TypedQuery<DashboardDTO> query = entityManager.createQuery(hql, Merchants.class);
        List<DashboardDTO> data = query.getResultList();

        return data;
    }

Java 对象：

public class DashboardDTO {

    private Date date;
    private int amount;
    private int number_of_transactions;

    public DashboardDTO(Date date, int amount, int number_of_transactions) {
        this.date = date;
        this.amount = amount;
        this.number_of_transactions = number_of_transactions;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    public int getAmount() {
        return amount;
    }

    public void setAmount(int amount) {
        this.amount = amount;
    }

    public int getNumber_of_transactions() {
        return number_of_transactions;
    }

    public void setNumber_of_transactions(int number_of_transactions) {
        this.number_of_transactions = number_of_transactions;
    }
}

如何正确实现查询？我想在不使用实体的情况下得到List<DashboardDTO> 的结果？

【问题讨论】：

您想执行原生查询并将结果映射到 java 对象？
是的——这就是我想要得到的结果。
请查看this 文章。特别是DTO projections using a ConstructorResult 部分

标签： java sql jpa spring-data-jpa

【解决方案1】：

简单的方法是将@Entity、@Id添加到您的DashboardDTO，并使您的本机sql别名与DTO的属性名称匹配。

@Entity
public class DashboardDTO {
  @Id
  private Date cdate;
  private int amount;
  private int numberOfTransactions;
  ...
}

public List<DashboardDTO> findAll() {
  String hql = "SELECT ... AS cdate, ... AS amount, ... AS number_of_transactions FROM ...";
  return entityManager.createNativeQuery(hql, DashboardDTO.class).getResultList();
}

【讨论】：