如何将Hibernate中的一对多关联添加到嵌入式类型？

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 ServiceCall s, 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.

Answer 2

What you need is the following:

• A Customer entity
• An embeddable ServiceCall
• A HistoricalServiceCall entity.

The Customer should contain an embedded ServiceCall field (the last service call).

The HistoricalServiceCall entity should have an ID, an embedded ServiceCall field (the data of the HistoricalServiceCall), and, potentially, a ManyToOne association to Customer if you want the OneToMany to be bidirectional.

The Customer should have a OneToMany association to HistoricalServiceCall.