Spring Data JPA-使用现有嵌套子对象持久化新对象

Question

我确实有两个实体，称为学校和水平，具有多对多的关系。

@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;

}

我的SchoolControllerRest controller有一个create方法，它接受一个SchoolDTO（目前它的结构与实体相同）。

当我试图通过JPARepository保存方法保存时，会抛出一个包含以下消息的异常：

"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:

通过遍历每个级别Id来解决这个问题...使用JPA存储库方法getOne（它调用EntityManager.getReference)获取引用，然后加载正确的模型，然后成功持久化。

@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());
    }

我的问题是：这是在Spring数据中发布关联的“唯一”正确方式吗？或者，有没有更好的办法？

Answer 1

在AddLevelTopackage函数中，为什么要使用mapschoolService.findbyid(id).map..，如果我认为schoolService将只返回具有给定id的实体学校的一个实例。我想用下面的地图代替地图就足够了：

...
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));
...