Erro em relacionamento Manytomany no Hibernate

Question

I have a problem with @manyToMany. You’re making the mistake:

org.hibernate.Multiplebagfetchexception: cannot simultaneously fetch Multiple bags

Follows my model:

@Entity
@Table(name = "sar_evento", schema = "sar")
@SequenceGenerator(name = "sequence", sequenceName = "sar.evento_sequence", schema = "sar")
public class Evento implements Serializable, BaseEntity {

    @Id
    @GeneratedValue(generator = "sequence", strategy = GenerationType.AUTO)
    @Column(name = "id_evento", nullable = false, unique = true)
    private Long id;

    @Column(length = 255)
    private String nome;

    @Column(columnDefinition = "text")
    private String descricao;

    @Column(name = "data_inicio")
    @Temporal(TemporalType.TIMESTAMP)
    private Date dataInicio;

    @Column(name = "data_fim")
    @Temporal(TemporalType.TIMESTAMP)
    private Date dataFim;

    @Column(name = "dia_todo")
    private boolean diaTodo;

    @ManyToOne
    @JoinColumn(name = "id_sala", referencedColumnName = "id_sala")
    private Sala sala;

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
    @OrderBy("nome asc")
    @JoinTable(name = "sar_evento_recurso", schema = "sar",
            joinColumns = {
                @JoinColumn(name = "id_evento")},
            inverseJoinColumns = {
                @JoinColumn(name = "id_recurso")})
    private List<Recurso> recursos = new ArrayList<Recurso>();

    @ManyToOne
    @JoinColumn(name = "id_usuario", referencedColumnName = "id")
    private Usuario usuario;

someone’s been through this trouble?

Answer 1

This error refers to cases where a query performed in JPA or Hibernate uses the fetch Join in more than one relationship.

The joins "normal", Lazy and Eager, bring the related lists in commands SELECT subsequent to the SELECT main, ie, it retrieves first the requested entity, then the data of your relationships.

This means that, in terms of performance, the mode Lazy and the way ager are equivalent. The difference is that the Lazy expects a method getter be invoked to execute the other queries while in mode ager they are executed right after the main.

Already the fetch Join force a Join native to the database, already retrieving both the main entity and related entities. This is usually used for optimization purposes.

However, Hibernate only supports performing the fetch Join in a related table. If you try to do more than one, the indicated error will occur.

The reason is quite simple when you think about what the result of query multi-jointed.

For example, let’s assume that you try to retrieve a list from the entity Evento and does the fetch Join with Recurso. If there is N events and M resources for each event the query will return N x M results, in which data from Evento will be repeated for each record of the respective Recurso.

Now, if we try to add any more relationships, for example Sala, your values will also be repeated N x M x K times.

In addition to the performance of the query starting to look bad, another problem emerges: it is very complicated to process the result of the query and identify which values repeat and which should create another entity in the list.

The solution is to recover these relationships in separate queries and not try to do everything at once.