Spring Data JPA - Persisting new Object with existing nested child

Question

I do have two entities called School and Level with a Many-To-Many relationship.

@Data
@Entity
public class School {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "school_generator")
    @SequenceGenerator(name="school_generator", sequenceName = "school_seq", allocationSize=1)
    @Column(name = "school_id")
    private Long id;

    @NotBlank
    private String name;

    @NotBlank
    private String city;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(
            name = "school_level",
            joinColumns = @JoinColumn(name = "school_id"),
            inverseJoinColumns = @JoinColumn(name = "level_id"))
    private Set<Level> levels = new HashSet<>();

}

@Data
@Entity
public class Level implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "level_generator")
    @SequenceGenerator(name = "level_generator", sequenceName = "level_seq", allocationSize = 1)
    @Column(name = "level_id")
    private Long id;

    @NotBlank
    @Column(nullable = false, unique = true)
    private String name;

}

My SchoolController , Rest controller, has a create method which takes a SchoolDTO (for now it has the same structure as the entity).

When I post the DTO, the nested child list Levels are populated only with existing Levels ids (since Levels are created previously).

When I try to save through the JPARepository save method...An exception with this message is thrown:

"detached entity passed to persist: com.salimrahmani.adawat.domain.Level; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: com.salimrahmani.adawat.domain.Level",
    "trace": "org.springframework.dao.InvalidDataAccessApiUsageException: detached entity passed to persist:

The problem is solved by iterating through each Level Id ...get the references by using the JPA Repository method getOne (which calls entityManager.getReference ) and then loads the right model and then persist successfully.

@PostMapping("/{id}/grades")
public ResponseEntity<SchoolDTO> addLevelToPackage(@PathVariable Long id, @RequestBody @Valid LevelDTO levelDTO) {
        return schoolService.findById(id)
                .map(school -> {
                    Level level = levelMapper.map(levelDTO);

                    if(level.getId() != null) {
                        level = levelService.getOne(level.getId());
                        school.getLevels().add(level);
                    } // else error

                    School saved = schoolService.save(school);

                    return ResponseEntity.ok(schoolMapper.map(saved));

                }).orElseGet(() -> ResponseEntity.notFound().build());
    }

My question is: Is this the "only" correct way to posting association in Spring Data? Or, is there any better way?

Answer 1

In the addLevelToPackage function Why using the map schoolService.findById(id).map.. if as I think the schoolService will return only one instance of the entity School with the given id. I think replacing the map with the following would be enough:

...
School theSchool = schoolService.findById(id);
Level theLevel = levelService.getOne(levelDTO.getId());
theSchool.getLevels().add(theLevel);
School saved = schoolService.save(theSchool);
return ResponseEntity.ok(schoolMapper.map(saved));
...