Question

I have an Entity that holds the last instance of a Component:

@Entity
public class Customer {
  ...
  @Embedded
  public ServiceCall getLastServiceCall() {...}
  ...
}

@Embeddable
public class ServiceCall() {
  public Date getDate();
  public Status getStatus();
}

The ServiceCall is embedded in order to call customer.getLastServiceCall().getDate() without requiring another table join.

I want to make that association one-to-many, and start saving all ServiceCalls, while holding one embedded in the customer.

Hibernate’s Docs (v3.6, which I’m using) states:

You can also use association annotations in an embeddable object (ie @OneToOne, @ManyToOne, @OneToMany or @ManyToMany). To override the association columns you can use @AssociationOverride.

and it seem that all I should do is add @OneToMany to the LastServiceCall association.

Will that work for me? If not, what are my alternatives? If yes, how will that affect 2nd level cache, and is there a limitation on updating that embedded instance (I can live with an immutable objects)?

Answer 1

@Embeded types are not supposed to have their own identity in the database, so I don’t think you can add @OneToMany to the Customer class on the ServiceCall.

  @OneToMany 
  @Embedded
  public ServiceCall getLastServiceCall() {...}

However you can add an association to the @Embeded Service call element like so.

@Entity 
pubic class HistoricalServiceCall extends ServiceCall 
{
   @Id 
   private String id; 

}


@Embeddable 
public class ServiceCall {

   @OneToMany(fetch=FetchType.LAZY)
   @JoinColumn(name="join_column_defined_on_customer_table")
   List<HistoricalServiceCall> getServiceCallHistory(); 
}

Update: putting FetchType.LAZY on the getServiceCallHistory() is a hint to the JPA provider to wait until you call getServiceCallHistory before it does another select to pull in that association.

So with the setup I am describing if you do customer.getLastServiceCall().getDate() it will not pull
in the ServiceCallHistory before the relationship is lazy.