Java Spring 投影内投影

Question

Is it possible to use a projection and in some related object use it's own projection?

For example, a have Exam , that has List<Question> . I'd like to request a list of exams (which I have a @projection ), but I'd like to define the attributes to be retrieved for each related Question

Answer 1

If I understand correctly you want to use Projection as children of Projection. If it is the case, yes, you can. You can create a QuestionProjection and use inside the ExamProjection.

Example:

@Projection(name = "questionProjection", types = { Question.class }) 
public interface QuestionProjection {
    // Getters
}

@Projection(name = "examProjection", types = { Exam.class }) 
public interface ExamProjection {
    List<QuestionProjection> getQuestionList();

    // Other Getters
}

Answer 2

You can do it something like this:

Assuming that your Exam entity might be:

@Entity
@Table(name = "EXAM")
public class Exam implements Serializable {

    private static final long serialVersionUID = 1L;

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

    @Column(name = "DESCR")
    private String descr;

    @OneToMany(mappedBy = "exam")
    private List<Question> questions;

      // Getters and Setters

and your Question entity

@Entity
@Table(name = "QUESTION")
public class Question implements Serializable {

    private static final long serialVersionUID = 1L;

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

    @Column(name = "DESCR")
    private String descr;

    @Column(name = "CONT")
    private String cont;

    @ManyToOne
    @JoinColumn(name = "EXAM_ID")
    @JsonIgnoreProperties("questions")
    private Exam exam;

So, create projections

public interface ExamProjection {
    Long getId();

    String getDescr();

    List<QuestionProjection> getQuestions();
}

and

public interface QuestionProjection {
    Long getId();

    String getDescr();
}

Your repository

@Repository
public interface ExamRepository extends JpaRepository<Exam, Long> {

    List<Exam> findAll();

    @Query("SELECT e FROM Exam e")
    List<ExamProjection> findAllProjection();
}

Observe that using the findAll() method and passing the list type as ExamProjection, for some reason, causes a incompatible return type error. To avoid that, create a custom method, in this case, findAllProjection() .

The service

@Service
public class ExamService {

    @Autowired
    ExamRepository examRepository;

    public List<ExamProjection> findAllProjection() {
        return examRepository.findAllProjection();
    }
}

and finally, the resource

@RestController
@RequestMapping(value = "/exam")
public class ExamResource {

    @Autowired
    ExamService examService;

    @GetMapping
    public ResponseEntity<List<ExamProjection>> findAll() {
        return ResponseEntity.ok().body(examService.findAllProjection());
    }
}

Using the above, the json returned don't contains the field cont , because de QuestionProjection don't have the method getCont() .

[
    {
        "id": 1,
        "descr": "First Exam",
        "questions": [
            {
                "id": 1,
                "descr": "First Question"
            },
            {
                "id": 2,
                "descr": "Second Question"
            }
        ]
    }
]

If the QuestionProjection changes to

public interface QuestionProjection {
    Long getId();

    String getCont();
}

the json returned changes to

[
    {
        "id": 1,
        "descr": "First Exam",
        "questions": [
            {
                "id": 1,
                "cont": "First Question Content"
            },
            {
                "id": 2,
                "cont": "Second Question Content"
            }
        ]
    }
]