JPA ManyToMany, how can JoinTable have a property?

Question

I have a question for designing the ManyToMany in EJB, how can a jointable has a property?
Here is an example, the students and courses are ManyToMany, every student hav

Answer 1

It is possible.

You just need to replace many-to-many mapping with the explicit combination of one-to-many and many-to-one mappings via a 3rd entity, that would represent the association between the two primary entities (student and course in your example).

Please read details here

Answer 2

It is not possible, you cannot add property to relationship. If you need to access property in the join table, then that property belongs to some entity and as a result you need third entity.

Answer 3

Entities with Many to Many Relationships (Merchant and Service). This can be achieved using a third entity as follow:-

@Entity
@Table(name = "merchant")
public class Merchant implements java.io.Serializable {

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.merchant",targetEntity = MerchantService.class)
    private Set<MerchantService> merchantServices = new HashSet<>();
}

@Entity
@Table(name = "merchant_service")
@AssociationOverrides({
        @AssociationOverride(name = "pk.merchant",
            joinColumns = @JoinColumn(name = "merchant_id")),
        @AssociationOverride(name = "pk.service",
            joinColumns = @JoinColumn(name = "service_id")) })
public class MerchantService implements java.io.Serializable {

    @EmbeddedId
    private MerchantServiceId pk = new MerchantServiceId();

    private boolean isActive;

    public MerchantServiceId getPk() {
        return pk;
    }

    public void setPk(MerchantServiceId pk) {
        this.pk = pk;
    }

    @Transient
    public Service getService() {
        return getPk().getService();
    }


    @Transient
    public Merchant getMerchant() {
        return getPk().getMerchant();
    }


    public boolean isActive() {
        return isActive;
    }

    public void setActive(boolean isActive) {
        this.isActive = isActive;
    }

}

@Embeddable
public class MerchantServiceId implements java.io.Serializable {

    private Merchant merchant;
    private Service service;

    @ManyToOne
    public Merchant getMerchant() {
        return merchant;
    }

    @ManyToOne
    public Service getService() {
        return service;
    }

}

@Entity
@Table(name = "service")
public class Service implements java.io.Serializable {

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.service",targetEntity = MerchantService.class)
    private Set<MerchantService> merchantServices = new HashSet<>();

}