Spring JPA native query with Projection gives “ConverterNotFoundException”

问题

I'm using Spring JPA and I need to have a native query. With that query, I need to get only two fields from the table, so I'm trying to use Projections. It isn't working, this is the error I'm getting:

org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [org.springframework.data.jpa.repository.query.AbstractJpaQuery$TupleConverter$TupleBackedMap] to type [com.example.IdsOnly]

I tried to follow precisely the instructions of that page I linked, I tried to make my query non-native (do I actually need it to be native if I use projections, btw?), but I always get that error.
If I use an interface it works, but the results are proxies and I really need them to be "normal results" that I can turn into json.

So, here's my code. The Entity:

import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@Entity
@Table(name = "TestTable")
public class TestTable {

    @Id
    @Basic(optional = false)
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "Id")
    private Integer id;
    @Column(name = "OtherId")
    private String otherId;
    @Column(name = "CreationDate")
    @Temporal(TemporalType.TIMESTAMP)
    private Date creationDate;
    @Column(name = "Type")
    private Integer type;
}

The class for the projection:

import lombok.Value;

@Value // This annotation fills in the "hashCode" and "equals" methods, plus the all-arguments constructor
public class IdsOnly {

    private final Integer id;
    private final String otherId;
}

The Repository:

public interface TestTableRepository extends JpaRepository<TestTable, Integer> {

    @Query(value = "select Id, OtherId from TestTable where CreationDate > ?1 and Type in (?2)", nativeQuery = true)
    public Collection<IdsOnly> findEntriesAfterDate(Date creationDate, List<Integer> types);
}

And the code that tries to get the data:

@Autowired
TestTableRepository ttRepo;
...
    Date theDate = ...
    List<Integer> theListOfTypes = ...
    ...
    Collection<IdsOnly> results = ttRepo.findEntriesAfterDate(theDate, theListOfTypes);  

Thanks for the help. I really don't understand what I'm doing wrong.

回答1:

The query should be using a constructor expression:

@Query("select new com.example.IdsOnly(t.id, t.otherId) from TestTable t where t.creationDate > ?1 and t.type in (?2)")

And i dont know Lombok, but make sure there is a constructor that takes the two IDs as parameters.

回答2:

with spring data you can cut the middle-man and simply define

public interface IdsOnly {
  Integer getId();
  String getOtherId();
}

and use a native query like;

@Query(value = "Id, OtherId from TestTable where CreationDate > ?1 and Type in (?2)", nativeQuery = true)
    public Collection<IdsOnly> findEntriesAfterDate(Date creationDate, List<Integer> types);

check out https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#projections

回答3:

JPA 2.1 introduces an interesting ConstructorResult feature if you want to keep it native.

回答4:

You can return list of Object Array (List) as return type of the native query method in repository class.

@Query(
            value = "SELECT [type],sum([cost]),[currency] FROM [CostDetails] " +
                    "where product_id = ? group by [type],[currency] ",
            nativeQuery = true
    )
    public List<Object[]> getCostDetailsByProduct(Long productId);

for(Object[] obj : objectList){
     String type = (String) obj[0];
     Double cost = (Double) obj[1];
     String currency = (String) obj[2];
     }

回答5:

I need to map values from 2 tables and solved like this in Repository file

@Query("select distinct emp.user.id as id, " +
    "concat(emp.user.firstName, ' ',emp.user.lastName, ' ',emp.extensionNumber) as name from NmsEmployee emp " +
    " where emp.domain.id = :domainId and (emp.activeUntil= null or emp.activeUntil > :lLogin) ")
    List<Selectable> getSelcByDomainId( @Param("domainId") Long domainId, @Param("lLogin") ZonedDateTime lLogin);

Where Selectable is

public interface Selectable {
     Long getId();
     String getName();
}

来源：https://stackoverflow.com/questions/49500309/spring-jpa-native-query-with-projection-gives-converternotfoundexception