如何在Hibernate / JPA中实现“可选”关系？

Question

I'm using the Hibernate 3.6 implementation of JPA. I have two tables in a logical parent-child relationship. Normally there would be one record in the "child" for each record in the "parent." The tables look like this:

CREATE TABLE FOO (
  FOO_ID INT IDENTITY,
  BAR_ID INT DEFAULT 0 NOT NULL,
  SOMEDATA CHAR(50) );

CREATE TABLE BAR (
  BAR_ID INT IDENTITY,
  OTHERDATA CHAR(50),
  STILLOTHER CHAR(50)) ;

However, it has been decided that some "parent" records should not have "child" records and I should just put a 0 in the FOO.BAR_ID column.

How do I implement this in my entity classes? Right now the classes look like this:

public class Foo implements java.io.Serializable {
    @Id
    @GeneratedValue
    @Column(name = "FOO_ID", unique = true, nullable = false)
    private Long fooId;
    @OneToOne(cascade = {CascadeType.MERGE, CascadeType.PERSIST})
    @JoinColumn(name = "BAR_ID", nullable = false)
    private Bar bar;
    }


public class Bar implements java.io.Serializable {
    @Id
    @GeneratedValue
    @Column(name = "BAR_ID", unique = true, nullable = false)
    private Long barId;

)

I can't seem find the right syntax to place a 0 in the FOO.BAR_ID column and not try to create a record in the BAR table. Can this even be done with Hibernate?

Answer 1

@OneToOne(cascade = {CascadeType.MERGE, CascadeType.PERSIST})
@JoinColumn(name = "BAR_ID", nullable = false)

I believe you should try removing.

nullable = false

You are telling hibernate that Foo's Bar can not be null. If Bar is optional then sometimes it must be null. Since it can not be null and the CascadeType is Persist, Hibernate is saving the Bar that Foo must have when you save Foo.