@OneToOne, unique constraint and redundant? index

hsiliev · **Joined:** Sun Jun 20, 2010 5:13 pm **Posts:** 3

I have these two classes:

Code:

@Entity
public class Parent implements Serializable {
  @Id
  @GeneratedValue
  private long id;

  @OneToOne(cascade = CascadeType.ALL)
  @PrimaryKeyJoinColumn
  private Child child;
}

and

Code:

@Entity
public class Child implements Serializable {
  @Id
  @GeneratedValue
  long id;

  @OneToOne(cascade = CascadeType.ALL, optional = true)
  private Parent parent;
}

The mapping column "parent_id" in the Child table is not unique. How can I make it unqiue?

If I specify @UniqueConstraint there are 2 indexes in the DB (MySQL) - one from OneToOne relation and one from the constraint. If I go for the foreign key approach with @JoinColumn(unique=true) the MySQL again contains index and unique index.

sanne.grinovero · **Posted:** Mon Jun 21, 2010 3:45 am

you're missing the "mappedBy" property of the @OneToOne annotation, so you're modeling two different relations instead of one bi-directional relation; because of that nothing is stopping the current model from having two Childs pointing to the same Parent.

hsiliev · **Joined:** Sun Jun 20, 2010 5:13 pm **Posts:** 3

You mean adding mappedBy toghether with @PrimaryKeyJoinColumn, or together with @JoinColumn (and FK)?

I'm asking because I already tried @JoinColumn(name="...", unique=true) with mappedBy on the other size and the result was as one would expect - foreign key mapping, but again there were 2 indexes.

sanne.grinovero · **Posted:** Mon Jun 21, 2010 7:05 am

AFAIR you won't need neither @PrimaryKeyJoinColumn nor @JoinColumn, make it working in the simple case first and then see if you still need to "tune" settings.
Just using the 2x@OneToOne properly (having one side marked by mappedBy) should create the effect you're looking for, as far as I understood.

hsiliev · **Joined:** Sun Jun 20, 2010 5:13 pm **Posts:** 3

@PrimaryKeyJoinColumn was causing incorrect behavior - Parent.getChild would return wrong object - Child with the same id as Parent (due to the performed join I think). Since the relation is 0-1 that's obviously not correct, so I removed it.

I've removed all additional annotations and set mappedBy on the Parent object. Now I have mapping column on Child object, however no unique constraint is created. Is this the intended behavior?