Association class Mapping in Hibernate

Question

I'm a student and I'm new in Hibernate framework, I have a association class and I want to know how to map it the Situation :

         Entity_1[0..\*]-------------------------[0..\*]Entity_2

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                          Association class
                           Date_affectation
                           Date_expiration

What the association rule should I use ?

Answer 1

You can model the association as a separate Association entity. The temporal data can be modelled as regular Date attributes (as @Temporal(TemporalType.TIMESTAMP)).

You can model the PK of an association as the composition of the foreign keys of Entity_1 and Entity_2 - this would make the association a dependent entity. Or you can assign it an own id and connect to Entity_1 and Entity_2 via ManyToOne relationships.

EDIT: The bulk of my example implementation is shamelessly stolen from @Vlad's answer, since the main difference is that an entity has an id and is queryable while an embeddable is not.

I have removed @Column(updatable=false) from the Date columns, because I am not sure that this is required and added @Temporal(TemporalType.TIMESTAMP) because it is required by the JPA spec and thus increases portability.

Since this is a bidirectional mapping, I use mappedBy on the inverse side (the entities) instead of JoinColumn on the owning side (the association).

@Entity
public class Association {

  @Id
  @GeneratedValue(...)
  private Long id;

  @Column
  @Temporal(TemporalType.TIMESTAMP)
  private Date affectation;

  @Column
  @Temporal(TemporalType.TIMESTAMP)
  private Date expiration;

  @ManyToOne(fetch=FetchType.LAZY)
  private Entity1 entity1;

  @ManyToOne(fetch=FetchType.LAZY)
  private Entity2 entity2;
}

Entity1 and Entity2 look the same, just replace the digit. I am not sure about the added value of using the Set interface, so I replaced it with List, but perhaps I am missing something.

public class Entity1 {

  @OneToMany(mappedBy="entity1")
  private List<Association> associations = new ArrayList<Association>();
  ...
}

Answer 2

Unless you ever want to fetch the Association by its id, you should choose to have an Embeddable Association which will define those two Date fields.

@Embeddable
public class Association {

    @Column(updatable=false)
    private Date affectation;

    @Column(updatable=false)
    private Date expiration;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="ENTITY_1_ID")
    private Entity1 entity1;

    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="ENTITY_2_ID")
    private Entity2 entity2;

}

public class Entity1 {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="ASSOCIATION"),
        joinColumns=@JoinColumn(name="ENTITY_1_ID")
    )
    private Set<Association> associations = new HashSet<Association>();
}

public class Entity2 {

    @CollectionOfElements
    @JoinTable(
        table=@Table(name="ASSOCIATION"),
        joinColumns=@JoinColumn(name="ENTITY_2_ID")
    )
    private Set<Association> associations = new HashSet<Association>();
}

This was previously explained here.