Fetch data for non table entity in spring data jpa

Question

I have user table with column id, mname, firstname, lastname, age (5 column) using spring data jpa I need to get only id, firstname, age (only 3 column)

userentity is below user.java

 @Entity
    @Table(name = "user")
   public class User implements Serializable {

    private static final long serialVersionUID = -3009157732242241606L;
    @Id
    private long id;

    @Column(name = "mname")
    private String mName;

    @Column(name = "firstname")
    private String firstName;

    @Column(name = "firstname")
    private String firstName;

    @Column(name = "lastname")
    private String lastName;

    @Column(name="age")
    private int age;

    //constructor with fileds


user2.java contains the fileds

private Long id;
private String firstname;
private int age;

//getter and setter

UserRepository.java

public interface UserRepository extends CrudRepository<User, Long> {
@Query(value="SELECT usr.id as id,usr.firstname as firstName, usr.age as age FROM user usr WHERE usr.id=?1", nativeQuery=true)
List<User2> getUserDetailsByUserId(Long id);
}

getting the below error in colsole:

org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.math.BigInteger] to type [com.user.entity.User2]

while inspect the code get the error:

Failed to convert from type [java.lang.Object[]] to type [com.user.entity.User2] for value '{403, firstnnnnnnn, 26}';

Is there any way to map the values to user2 (non-table entity)?

Answer 1

You should use projections :

Interface based :

public interface UserProjection {
    Long getId();
    String getFirstname();
    Integer getAge();
}

public interface UserRepository extends CrudRepository<User, Long> {
    List<UserProjection> findById(Long id);
}

Class based (DTO) :

@Value // It's Lombok annotation
public class UserDto {
    Long id;
    String firstname;
    Integer age;
}

public interface UserRepository extends CrudRepository<User, Long> {
    List<UserProjection> findById(Long id);
    List<UserDto> getById(Long id);
}

Answer 2

No, In reality your query return 3 attributes id, firstName, age and not an Object User2 , you can solve your problem like :

First Solution

You can create a constructor in your User2 class which hold this three attributes :

public User2(Long id, String firstName, int age){..}

Then your query should look like this :

SELECT com.packagename.User2(usr.id, usr.firstname, usr.age) FROM user usr WHERE usr.id=?1
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

---

Second Solution

You have to change :

List<User2> getUserDetailsByUserId(Long id);
     ^^^^^

to this :

List<Object[]> getUserDetailsByUserId(Long id);
     ^^^^^^^^

Then you can loop over the result in your service and cast each Object like this :

List<Object[]> listObjects = userRepository.regetUserDetailsByUserId(id);
List<User2> listUsers = new ArrayList<>();
for(Object[] obj : listObjects){
   listUsers.add((Long)obj[0], (String)obj[1], (Integer)obj[2]);
}