views:

138

answers:

1

What's the exact difference between @JoinColumn and @PrimaryKeyJoinColumn?

You use @JoinColumn for columns that are FKs. A typical column could look like (e.g. in a join table with additional attributes):

@ManyToOne
@JoinColumn(name = "...")
private OtherClass oc;

What happens if I promote the column to be a/the PK, too (a.k.a. identifying relationship)? As the column is now the PK, I must tag it with @Id:

@Id
@ManyToOne
@JoinColumn(name = "...")
private OtherClass oc;

Now the question is: are @Id + @JoinColumn the same as just @PrimaryKeyJoinColumn?:

@ManyToOne
@PrimaryKeyJoinColumn(name = "...")
private OtherClass oc;

If not, what's @PrimaryKeyJoinColumn there for?

A: 

What happens if I promote the column to be a/the PK, too (a.k.a. identifying relationship)? As the column is now the PK, I must tag it with @Id (...).

This enhanced support of derived identifiers is actually part of the new stuff in JPA 2.0 (see the section 2.4.1 Primary Keys Corresponding to Derived Identities in the JPA 2.0 specification), JPA 1.0 doesn't allow Id on a OneToOne or ManyToOne. With JPA 1.0, you'd have to use PrimaryKeyJoinColumn and also define a Basic Id mapping for the foreign key column.

Now the question is: are @Id + @JoinColumn the same as just @PrimaryKeyJoinColumn?

You can obtain a similar result but using an Id on OneToOne or ManyToOne is much simpler and is the preferred way to map derived identifiers with JPA 2.0. PrimaryKeyJoinColumn might still be used in a JOINED inheritance strategy. Below the relevant section from the JPA 2.0 specification:

11.1.40 PrimaryKeyJoinColumn Annotation

The PrimaryKeyJoinColumn annotation specifies a primary key column that is used as a foreign key to join to another table.

The PrimaryKeyJoinColumn annotation is used to join the primary table of an entity subclass in the JOINED mapping strategy to the primary table of its superclass; it is used within a SecondaryTable annotation to join a secondary table to a primary table; and it may be used in a OneToOne mapping in which the primary key of the referencing entity is used as a foreign key to the referenced entity[108].

...

If no PrimaryKeyJoinColumn annotation is specified for a subclass in the JOINED mapping strategy, the foreign key columns are assumed to have the same names as the primary key columns of the primary table of the superclass.

...

Example: Customer and ValuedCustomer subclass

@Entity
@Table(name="CUST")
@Inheritance(strategy=JOINED)
@DiscriminatorValue("CUST")
public class Customer { ... }

@Entity
@Table(name="VCUST")
@DiscriminatorValue("VCUST")
@PrimaryKeyJoinColumn(name="CUST_ID")
public class ValuedCustomer extends Customer { ... }

[108] The derived id mechanisms described in section 2.4.1.1 are now to be preferred over PrimaryKeyJoinColumn for the OneToOne mapping case.

See also


This source http://weblogs.java.net/blog/felipegaucho/archive/2009/10/24/jpa-join-table-additional-state states that using @ManyToOne and @Id works with JPA 1.x. Who's correct now?

The author is using a pre release JPA 2.0 compliant version of EclipseLink (version 2.0.0-M7 at the time of the article) to write an article about JPA 1.0(!). This article is misleading, the author is using something that is NOT part of JPA 1.0.

For the record, support of Id on OneToOne and ManyToOne has been added in EclipseLink 1.1 (see this message from James Sutherland, EclipseLink comitter and main contributor of the Java Persistence wiki book). But let me insist, this is NOT part of JPA 1.0.

Pascal Thivent
This sourcehttp://weblogs.java.net/blog/felipegaucho/archive/2009/10/24/jpa-join-table-additional-statestates that using @ManyToOne and @Id works with JPA 1.x. Who's correct now?
Kawu
@Kawu See my update.
Pascal Thivent
OK. Thanks for clearing that up. I am right that the bad example a referred to uses @IdClass wrongly? Shouldn't the @Id annotations be placed at the separate (redundant/duplicate) columns in the entity class to be correct? (ven though I know that using @IdClass is not recommended anymore)
Kawu
I mean, there should be two properties in the class: @Id @Column private String institution; and @Id @Column private String competition; just to specify the PK right?
Kawu
@Kawu I'm sorry but honestly, it's too hard to discuss this in a small comment box. I'm not even sure I understood what you're talking about. If you have another specific question, I suggest to either pick a JPA implementation and to experiment a bit or to post a *new* question with a full example (and the JPA version). This would make things much easier.
Pascal Thivent