使用带有Spring Data JPA的自定义查询返回自定义对象

Question

I have researched question and couldn't find appropriate answer.

I am trying to return only certain columns from table using custom query with Spring Data JPA in my Spring Rest application. However query always throws exception when executed.

org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [org.forum.api.model.Message]

I know that it is possible to use String but why are Message objects not Serialized properly into JSON even thought I have created model for it in a sub-package of Spring Boot main ?

Here is my model class.

@Entity
@Table(name="message")
public class Message {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "text_id")
    private long id;

    @NotNull
    private String author;

    @NotNull
    private String text;

    @NotNull
    private String recepient;

    public long getId() {return id;}

    public void setId(long id) {this.id = id;}

    public String getAuthor() {return author;}

    public void setAuthor(String author) {this.author = author;}

    public String getText() {return text;}

    public void setText(String text) {this.text = text;}

    public String getRecepient() {return recepient;}

    public void setRecepient(String recepient) {this.recepient = recepient;}

}

Here is controller class.

@RestController
@RequestMapping("/api")
public class MessageController {

    @Autowired
    private MessageService messageService;

    @GetMapping("/message/{id}")
    public Message getMessageTextById(@PathVariable(value="id") Long id) {
        return messageService.getMessageTextById(id);       
    }

}

Here is service class.

@Service
public class MessageServiceImpl implements MessageService {

    @Autowired
    MessageRepository messageRepo;

    @Override
    public Message getMessageTextById(Long id) {        
        return messageRepo.findMessageTextById(id);     
    }

}

Here is Repository Class

@Repository
public interface MessageRepository extends JpaRepository<Message, Long> {


    @Query("SELECT m.author, m.text FROM Message m WHERE m.id = :id")
    Message findMessageTextById(@Param("id") Long id);

}

Answer 1

If you want to retrieve only certain columns you can use a simple bean class:

public class CustomMessage{
  private String author;
  private String text;

  public CustomMessage(String author, String text) {
    this.author = author;
    this.author = text;
  }
}

Then return a bean instance from your repository:

@Query("SELECT new path_to_class.CustomMessage(m.author, m.text) FROM Message m WHERE m.id = :id")

Or retrieve a map:

 @Query("SELECT new map(m.author as author, m.text as text) FROM Message m WHERE m.id = :id")