Spring Data JPA：查询如何返回非实体对象或对象列表？

Question

I am using spring data JPA in my project. I am playing with millions of records. I have a requirement where I have to fetch data for various tables and build a object and then paint it on a UI. Now how to achieve this my Spring data Repositories. I have read that it can be achieved by Named native queries.

If the named native query does not return an entity or a list of entities, we can map the query result to a correct return type by using the @SqlResultSetMapping annotation.

But when I am trying to use @SqlResultSetMapping it is taking another entityResult . Mean what I understand is that it is just transformation some query result to entity result set only, but I want a result set of non - entities objects.

@SqlResultSetMapping(
    name="studentPercentile",
    entities={
        @EntityResult(
           entityClass=CustomStudent.class,
              fields={
                  @FieldResult(name="id", column="ID"),
                  @FieldResult(name="firstName", column="FIRST_NAME"),
                   @FieldResult(name="lastName", column="LAST_NAME")
              }         
        )
   }
) 
@NamedNativeQuery(
    name="findStudentPercentile", 
    query="SELECT * FROM STUDENT", 
    resultSetMapping="studentPercentile")

In above example I am just trying to get a results from student Entity into another pojo ' CustomStudent ' which is not a entity. ( This example I am trying to execute just for POC purpose, actual usecase is much complicated, with complicated query returning different resultset ).

How to achieve above usecase? Is there any other way besides using name query that my repository method returning Non - Entities objects?

Answer 1

You can do something like

@NamedQuery(name="findWhatever", query="SELECT new path.to.dto.MyDto(e.id, e.otherProperty) FROM Student e WHERE e.id = ?1")

Then the MyDto object would just need a constructor defined with the correct fields ie

public MyDto(String id, String otherProperty) { this.id = id; this.otherProperty = otherProperty; }

Answer 2

I was deeply surprised when I came accross this for the first time but, yes, you can map query results using @SqlResultSetMapping only to scalars and managed entities.

The best you can do, I guess, is to skip automatic mapping. Query without mapping would return List<Object[]> and you can map it the way you need.

Another approach would be to use @MappedSuperclass. The class denoted as @MappedSuperclass (CustomStudent in your case) can be (not sure 100%, though) used in @SqlResultSetMapping. but you need to introduce inheritance hierarchy, that is your Student entity must extend CustomStudent. That would suck most of the time from the proper OO design, because inheritance would be a little bit artificial...

Answer 3

How about JPA 2.1 ConstructorResult ?

@SqlResultSetMapping(
    name="studentPercentile",
    classes={
        @ConstructorResult(
            targetClass=CustomStudent.class,
            columns={
                @ColumnResult(name="ID"),
                @ColumnResult(name="FIRST_NAME"),
                @ColumnResult(name="LAST_NAME")
            }
        )
    }
)

@NamedNativeQuery(name="findStudentPercentile", query="SELECT * FROM STUDENT", resultSetMapping="studentPercentile")

Answer 4

We can also parse using JSON help.

Class level declaration.

@Autowired
private EntityManager em;
private ObjectMapper mapper = new ObjectMapper(); 

---

Main Code.

Query query = em.createNativeQuery(argQueryString);
NativeQueryImpl nativeQuery = (NativeQueryImpl) query;
nativeQuery.setResultTransformer(AliasToEntityMapResultTransformer.INSTANCE);
List<Map<String,Object>> result = nativeQuery.getResultList();
List<CustomStudent> resultList = result.stream()
   .map(o -> {
         try {
              return 
            mapper.readValue(mapper.writeValueAsString(o),CustomStudent.class);
       } catch (Exception e) {
           ApplicationLogger.logger.error(e.getMessage(),e);
       }
     return null;
    }).collect(Collectors.toList());