How to annotate 1 to 0..1?

Immo Heikkinen · **Posted:** Tue Oct 02, 2007 7:40 am

Consider the following entities:

Code:

@Entity
public class Parent {

   @Id
   private int id;
   
   @OneToOne
   private Child child;

   // ...

}


@Entity
public class Child {
   
   @Id
   private int id;

   // ...
}

I am using Hibernate to generate the database. What I want is the following:
- Parent and child are sharing the primary key
- Foreign key Child.id --> Parent.id will be created

I have tried the following additional annotations together with @OneToOne but no success:

1) No extra annotation: column Parent.child_id and foreign key Parent.child_id --> Child.id are created
2) @PrimaryKeyJoinColumn: no extra columns but no foreign keys either
3) @JoinColumn(name="id"): Same as 2)

Option 3) would do the job if the association was @OneToMany.

Any help would be greatly appreciated.

kimptoc · **Joined:** Tue Jun 27, 2006 6:14 am **Posts:** 24

Hi,

Not an answer to your question, more like fishing for an answer to mine...

I think we are trying to do something similar, although we pre-create the tables/relationships ourselves and just use hibernate for the data access.

We have a problem with how to navigate between the Parent/Child classes and specify the Id. See this entry: http://forum.hibernate.org/viewtopic.php?t=980020

So, do you need to get the Parent object, given a Child object, if so, how do you do it?

Thanks in advance,
Chris

Immo Heikkinen · **Posted:** Thu Oct 04, 2007 2:50 am

Hi there,

I want the association to be unidirectional so that Parent has a reference to Child but not the other way around. So I do not need to get the Parent object, given a Child object.

If you need to navigate both ways, you need to have an bidirectional association:

Code:

@Entity
public class Parent {

   @Id
   private int id;
   
   @OneToOne
   @JoinColumn(name="id")
   private Child child;

   // ...

}


@Entity
public class Child {
   
   @Id
   private int id;

   @OneToOne(mappedBy="child")
   private Parent parent;

   // ...
}

Unfortunately this doesn't help in my case since no foreign key will be created at all.

efriedman · **Joined:** Tue Aug 07, 2007 2:31 am **Posts:** 11

I think this does what you want.

Parent:

Code:

@Entity
public class Parent {

      @Id @GeneratedValue(strategy=GenerationType.AUTO)
      private int id;
      
      @OneToOne(mappedBy = "parent", fetch=FetchType.LAZY, cascade=CascadeType.ALL, optional=true)
      private Child child; 
}

Child with the same ID as the parent:

Code:

@Entity
public class Child {
   @Id @GeneratedValue(generator="parentPKGenerator")
   @org.hibernate.annotations.GenericGenerator(
         name = "parentPKGenerator",
         strategy ="foreign",
         parameters = @org.hibernate.annotations.Parameter(name = "property", value = "parent")
   )
   private int id;

   @OneToOne(optional=false, fetch=FetchType.LAZY)
   @PrimaryKeyJoinColumn
   private Parent parent; 
}

Generates the following for mysql:

Code:

    create table Child (
        id integer not null,
        primary key (id)
    ) type=InnoDB;

    create table Parent (
        id integer not null auto_increment,
        primary key (id)
    ) type=InnoDB;

    alter table Child 
        add index FK3E104FC5F807672 (id), 
        add constraint FK3E104FC5F807672 
        foreign key (id) 
        references Parent (id);

Immo Heikkinen · **Posted:** Sat Oct 06, 2007 10:23 am

Thanks, that does almost what I want.

I would still like the class-level association between Parent and Child to be unidirectional, i.e. get rid of the parent field in Child class. But I guess that it is needed to get the foreign key created.

Immo Heikkinen · **Posted:** Mon Oct 08, 2007 7:23 am

It seems like the FK constraint name cannot be changed in this solution. I've tried @org.hibernate.annotations.ForeignKey in both classes but it doesn't work.

Any ideas?

emmanuel · **Posted:** Fri Oct 19, 2007 4:53 pm

Post a JIRA issue for the FK name on a @OneToOne with your attached test case. This feature should be added