java - JPA:@JoinColumn 和 @PrimaryKeyJoinColumn 之间的区别？

Question

@JoinColumn 和 @PrimaryKeyJoinColumn 的具体区别是什么？

您将 @JoinColumn 用于作为外键一部分的列。典型的列可能如下所示(例如，在具有附加属性的连接表中):

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

如果我也将该列提升为 PK(也称为识别关系)，会发生什么情况？由于该列现在是 PK，我必须用 @Id 标记它:

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

现在的问题是:

@Id + @JoinColumn 是否与@PrimaryKeyJoinColumn 相同？:

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

如果不是，那 @PrimaryKeyJoinColumn 有什么用？

Answer 1

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 (...).

这种对派生标识符的增强支持实际上是new stuff in JPA 2.0 的一部分。 (请参阅 JPA 2.0 规范中的 2.4.1 对应于派生身份的主键部分)，JPA 1.0 不允许 OneToOne 上的 Id > 或 ManyToOne。使用 JPA 1.0，您必须使用 PrimaryKeyJoinColumn 并为外键列定义 Basic Id 映射。

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

您可以获得类似的结果，但在 OneToOne 或 ManyToOne 上使用 Id 会简单得多，并且使用 JPA 2.0 映射派生标识符的首选方式。 PrimaryKeyJoinColumn 可能仍用于 JOINED 继承策略。在 JPA 2.0 规范的相关部分下方:

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.}

另见

• Primary Keys through OneToOne Relationships

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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?

作者正在使用 EclipseLink 的预发布 JPA 2.0 兼容版本(本文发表时版本为 2.0.0-M7)来撰写有关 JPA 1.0(!) 的文章。这篇文章具有误导性，作者使用的东西不是 JPA 1.0 的一部分。

作为记录，在 EclipseLink 1.1 中添加了对 OneToOne 和 ManyToOne 的 Id 支持(参见 this message 中的 James Sutherland , EclipseLink committer 和 Java Persistence wiki book 的主要贡献者)。但让我坚持，这不是 JPA 1.0 的一部分。