How can I use generated value within composite keys?

Question

I have two classes documentlog and documentversion(with primary keys: int doc_id and int docVersionID) with a many-to-one relationship. I used a composite key class called CompundKey to manage the compound primary key. I need to auto increment docversionID but I am not able to do that. Could you please help me in this regard?

@Entity
@Table(name = "Documentversion", schema = "DocumentManagement")
public class DocumentVersion implements Serializable { 

 private CompoundKey id;
 private List<DocumentLog> documentLog;

 @OneToMany(mappedBy="documentVersion", targetEntity=DocumentLog.class,  
   cascade ={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH})
 public List<DocumentLog> getDocumentLog() {
  return documentLog;
 }
 public void setDocumentLog(List<DocumentLog> documentLog) {
  this.documentLog = documentLog;
 }

 @EmbeddedId 
 @AttributeOverride(name="doc_Id", column=@Column(name="doc_Id") )
 public CompoundKey getId() {
  return id;
 }
 public void setId(CompoundKey id) {
  this.id = id;
 } 
}

Diffusivity

Answer 1

The documentation is a bit confusing on this topic...

To my knowledge, composite keys always had to be assigned by the application (i.e. non generated) at least with standard JPA but also Hibernate Core:

8.4. Components as composite identifiers

...

You cannot use an IdentifierGenerator to generate composite keys. Instead the application must assign its own identifiers.

But things might be a bit different in practice (see HHH-2060 and/or this thread for an alternative using a CompositeUserType together with an IdentifierGenerator).

Now, the most confusing part, from the Hibernate Annotations 3.5 documentation:

2.2.3.2.4. Partial identifier generation

Hibernate supports the automatic generation of some of the identifier properties. Simply use the @GeneratedValue annotation on one or several id properties.

...

You can also generate properties inside an @EmbeddedId class.

(and please also read the warning from the Hibernate Team against using this feature).

I don't have any practical experience with it though.

References

• Hibernate Core Reference Guide
  • 8.4. Components as composite identifiers
• Hibernate Annotations 3.5 Reference Guide
  • 2.2.3.2.4. Partial identifier generation

Assisi

This area is completely clear as mud. Actually Hibernate has allowed generation of the entire component making up the composite key for quite some time; but only through hbm.xml mappings, we never exposed that through annotations. The notion of "partial identifier generation" comes from a, oh lets be nice and call it an "interesting" reading of the JPA spec by the SpecJ team. Essentially they claim that because the spec does not disallow it, it is legal to attach @GeneratedValue to one or more of the @Id mappings making of the composite id. Hope that helps. – Baines

Answer 2

It is possible to declared your own generator for @EmbeddedId to use sequence for Id generation.

Also, you'll need to declare fake entity to create sequence automatically.

Wolfe