Many-To-Many relationship with Attributes and unidirectional mapping

Question

I'm pretty new to Hibernate and after searching for a while, I still have problems with my annotations.

I have 3 classes:

1. AuftragsPosition
2. AuftragspositionZuordung (join table)
3. Material

I want a Many-To-Many relationship between AuftragsPosition and Material with an additional attribute on join table. The additional attribute menge should be accessable in AuftragsPosition . My Material should have no knowledge about the other two tables.

Here are my classes:

AuftragsPosition

@Entity
@Table(name = "AUFTRAGSPOSITION")
public class AuftragsPosition extends AbstractVersionedEntity<Long> {

    private List<Material> materialien;

    @OneToMany(mappedBy = "position", targetEntity = AuftragspositionZuordnung.class, cascade = CascadeType.ALL)
    public List<Material> getMaterialien() {
        return materialien;
    }
}

AuftragspositionZuordnung

@Entity
@Table(name = "ZUORDNUNG_AUFTRAGSPOSITION")
public class AuftragspositionZuordnung extends AbstractVersionedEntity<Long> {

    private AuftragsPosition position;

    private Material material;

    private Integer menge;

    @ManyToOne
    @JoinColumn(name = "AUFTRAGSPOSITION_ID")
    public AuftragsPosition getPosition() {
        return position;
    }

    @ManyToOne(targetEntity = Material.class)
    @JoinColumn(name = "MATERIAL_ID")
    public Material getMaterial() {
        return material;
    }

    @Column(name = "ANZAHL")
    public Integer getMenge() {
        return menge;
    }
}

Do you have any idea how I should annotate these classes?

Thanks and kind regards, Kevin

Answer 1

I would add a map to the AuftragsPosition class looking approximately like this:

@ElementCollection
@CollectionTable(name = "MENGEN",
        joinColumns = @JoinColumn(name = "AUFTRAGSPOS_ID"))
@MapKeyJoinColumn(name = "MATERIAL") 
@Column(name = "AMOUNT")
private Map<Material, Integer> amounts = new HashMap<Material, Integer>();

You need only the @ElementCollection annotation. I have included the others to demonstrate how you could manipulate the default table and column naming.

I have assumed that you want to key by Material , not by the amount - this would change the required annotations.

Answer 2

You could create a join class ( AuftragsPositionMaterial ) that can contain the extra attribute menge .

@Entity
@Table(name = "AUFTRAGSPOSITION")
public class AuftragsPosition extends AbstractVersionedEntity<Long> {

    private Set<AuftragsPositionMaterial> auftragsPositionMaterials = new HashSet<AuftragsPositionMaterial>();

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.auftragsposition", cascade=CascadeType.ALL)
    public Set<AuftragsPositionMaterial> getAuftragsPositionMaterials() {
        return auftragsPositionMaterials;
    }
}

The Join Class:

@Entity
@Table(name = "AuftragsPosition_Material")
@AssociationOverrides({
        @AssociationOverride(name = "pk.auftragsPosition", 
            joinColumns = @JoinColumn(name = "AUFTRAGSPOSITION_ID")),
        @AssociationOverride(name = "pk.material", 
            joinColumns = @JoinColumn(name = "MATERIAL_ID")) })
public class AuftragsPositionMaterial {

    private AuftragsPositionMaterialID pk = new AuftragsPositionMaterialID();

    @Column(name = "MENGE")
    private String menge;

    @EmbeddedId
    public AuftragsPositionMaterialID getPk() {
        return pk;
    }

}

Now the Primary Key:

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

    private AuftragsPosition auftragsPosition;
    private Material material;

    @ManyToOne
    public Stock getAuftragsPosition() {
        return auftragsPosition;
    }

    public void setAuftragsPosition(AuftragsPosition auftragsPosition) {
        this.auftragsPosition= auftragsPosition;
    }

    @ManyToOne
    public Material getMaterial() {
        return material;
    }

    public void setMaterial(Material material) {
        this.material= material;
    }

}