如何将 JPA 创建本机查询映射到投影

Question

我正在尝试使用 Spring Data JPA createNativeQuery 从 postgreSQL 数据库中获取计数。但是，查询返回的是 null 而不是实际值。

下面是 JPA createNativeQuery 语句：

Query q = entityManager.createNativeQuery("SELECT null AS id, 
count(case when (IS_QUERIED = false AND SUBMITTED = true) 
then 1 else null end) AS pending,  
count(case when ( (SUBMITTED = true)) then 1 else null end) AS submitted, 
count(*) AS totalApplications FROM ANNUAL_RETURNS ", AnnualReturn.class);
//Note: AnnualReturn is the name of the @Entity class

List <AnnualReturn> countList=q.getResultList();

return countList;

我需要帮助映射我的查询中的“已提交”、“待处理”和“totalApplications”实例，以返回如下结果。

预期结果是：

"data": {
        "totalApplications": 2000,
        "submitted": 560,
        "pending": 60,
    }

结果得到：

{
    "data": [
        null
    ]

我将不胜感激。

Answer 1

我不知道为什么，但我相信 SELECT null AS id 导致获取空记录。

如果您不想获取 id，则可以将投影与自定义 RowMapper 或 DTO 投影一起使用。

见下文：

@Entity
@Data
@ToString
class A {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    Long id;
    String name;
    int age;

    public A() {
    }
    public A(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

interface ARepo extends JpaRepositoryImplementation<A, Long> {
}

@Component
@RequiredArgsConstructor
class Init {
    final ARepo repo;
    final EntityManager em;

    @EventListener
    public void init(ContextRefreshedEvent evt) {
        repo.save(new A("XX", 5));
        repo.save(new A("ZZ", 6));
        repo.save(new A("AA", 11));
        repo.save(new A("AB", 12));

        Query q = em.createNativeQuery("select 0 as id, a.name as name, a.age as age  from A a where a.total > 10 ", A.class);

        System.out.println(q.getResultList()); //fetches AA and BB 
        //if you change the query to select null as id  it would return [ null, null]
          
    }
}

Answer 2

你不应该在这里使用类型化查询，试试类似

Query q = entityManager.createNativeQuery("SELECT " +
  " COALESCE(sum(case when (IS_QUERIED = false AND SUBMITTED = true) " +
  " then AMOUNT else 0 end),0) AS pending,  " +
  " COALESCE(sum(case when ( (SUBMITTED = true)) then 1 else 0 end),0) AS submitted, " +
  " COALESCE(sum(AMOUNT),0) AS totalApplications FROM ANNUAL_RETURNS ");

List <Object[]> countList=q.getResultList();

Object[] obj = countList.get(0);

for (Object value : obj) {
     System.out.println(value);
}
long pending = ((BigInteger)obj[0]).longValue();
long submitted = ((BigInteger)obj[1]).longValue();
long total = ((BigInteger)obj[2]).longValue();


return ...; // prepare the values to your convinience

编辑

将 null 更改为 0 以避免结果中可能出现 null 值

编辑

修复空表上的 NPE