通过获取分离实体以保持@ManyToMany双向关系

Question

I am having difficulty persisting entity with many-to-many relationship (bidirectional) with JPA. Below are the sample code:

@Entity
@Table(name = "aentity")
public class AEntity {
    @Id
    @Column(name = "id",
            unique = true)
    @TableGenerator(initialValue = 1,
            name = "aentity_id_generator",
            pkColumnName = "table_name",
            pkColumnValue = "aentity",
            table = "id_generator",
            valueColumnName = "id")
    @GeneratedValue(generator = "aentity_id_generator",
            strategy = GenerationType.TABLE)
    private BigInteger id;

    @JoinTable(name = "bentity_aentities")
    @ManyToMany
    private Set<BEntity> bentities;

    /* getters and setters follows */
}

@Entity
@Table(name = "bentity")
public class BEntity {
    @Id
    @Column(name = "id",
            unique = true)
    @TableGenerator(initialValue = 1,
            name = "bentity_id_generator",
            pkColumnName = "table_name",
            pkColumnValue = "bentity",
            table = "id_generator",
            valueColumnName = "id")
    @GeneratedValue(generator = "bentity_id_generator",
            strategy = GenerationType.TABLE)
    private BigInteger id;

    @ManyToMany(mappedBy = "bentities")
    private Set<AEntity> aentities;

    /* getters and setters follows */
}

Below is the dto to entity converter...

public class DtoToEntityConverter {
   public void convertToEntity(AEntityDto aDto, AEntity a) {
      a.setBEntities(aDto.getBEntities().parallelStream().
         .map(bDto -> {
            return toBEntity(bDto); //this will just copy/transfer the properties from bEntityDto to bEntity.
         })
         .collect(Collectors.toSet()));
   }
}

Scenario 1: Saving AEntity with BEntity (id = null) - OK

Scenario 2: Saving AEntity with existing BEntity (id = id existing in db)

The following exception occurs in Scenario 2: Been looking for same questions in stackoverflow and tried different combination and suggestion but with no lock.

detached entity passed to persist: BEntity; nested exception is org.hibernate.PersistentObjectException: detached entity passed to persist: BEntity" 

Can anybody help please. Thanks.

Answer 1

You can try...

In the second entity:

@ManyToMany(cascade=CascadeType.ALL, mappedBy="bentities")
private Set<AEntity> aentities;

In the first entity:

@ManyToMany(cascade=CascadeType.ALL) 
@JoinTable(name="bentity_aentities", joinColumns=@JoinColumn(name="aentity_id"), inverseJoinColumns=@JoinColumn(name="bentity_id")) 
private Set<BEntity> bentities;

Answer 2

Finally solved my problem. It was the issue in DTO to Entity converter (my bad).

Previous converter:

public AEntity toAEntity(@NotNull AEntityDto aentityDto) {
     AEntity aentity = new AEntity();
     copyProperties(aentityDto, aentity);
     return aentity;
}

Refactored converter: Return the reference of the existing entity.

public AEntity toAEntity(@NotNull AEntityDto aentityDto) {
     AEntity aentity = aRepository.findSingleByTitle(aentityDto.getTitle());
     if(aentity == null) {
         aentity = new AEntity();
         copyProperties(aentityDto, aentity);
     }
     return aentity;
}

Thanks.