JPA: @ManyToOne legacy mapping using @JoinTable

Question

Dear JEE experts,
I have a tough legacy mapping problem. There are two entities Pac and BasePac where each Pac has a BasePac field which is to be queried from the other entity table. The association is definied in a third table pac_component which has a Pac and a BasePac field among several others. I think the schema is a bit weird and I would have defined it differently, but I cannot change it because other applications using the database must not be changed.
My code looks like this:
@javax.persistence.Entity(name="Pacs")
@javax.persistence.Table(name="packet")
@javax.persistence.SequenceGenerator(name="PacsSeqGen", sequenceName="packet_packet_id_seq")
public class Pac
     implements java.io.Serializable
    // virtual attribute basePac
    @javax.persistence.ManyToOne(fetch=EAGER, optional=true) // optional should be default anyway
    @javax.persistence.JoinTable(
         name="packet_component",
         [email protected](name="packet_id"),
         [email protected](name="basepacket_id") )
    private BasePac basePac;
    public BasePac getBasePac() { return basePac; }
    public void setBasePac( BasePac basePac ) { this.basePac = basePac; }
@javax.persistence.Entity(name="BasePacs")
@javax.persistence.Table(name="basepacket")
@javax.persistence.SequenceGenerator(name="BasePacsSeqGen", sequenceName="basepacket_basepacket_id_seq")
public class BasePac
     implements java.io.Serializable
{ ... }The Entity for pac_component does not appear so far and afaik it does not matter.
When I now create a Pac instance and persist it, JPA (with Hibernate) always wants to create a link object:
insert into pac_component (basepacket_id, packet_id) values (?, ?)Where the ? for basepacket_id is null. But this is not a valid row for pac_component, thus I will get a ConstaintViolationException.
My question is: Why does it create this row at all? The association is marked optional!
My solution might be to make the field PacBase within Pac transient and access it only through a pacComponents field, which is a @OneToMany but every assiciated PacComponent entity refers to the same BasePac. Anyway, I wonder why JPA or Hibernate wants to create such a row at all.
... MIchael

Answer 1

I wouldn't focus too much on wanting to solve this through JPA. What you have here is a 'problem' which you will run into in many forms - your business requirements do not map directly to the data layer. This simply means that you need some business logic to make the translation. For example if this were for a web layer I would implement a specialized bean which can take different entities and then provide an alternative view on the data, optionally by generating it.
If 'calculated' data is closely tied to the database layer and less to the business layer then you could of course choose to fix it through the database itself - by creating a view and mapping an entity to that. That is especially useful if you need the same data in multiple aspects of the application framework and not only in the Java code (think of reporting and analysis for example), but it has other considerations like performance.