如何在JPA中创建包含@ManyToOne属性作为@EmbeddedId的复合主键? [英] How to create a composite primary key which contains a @ManyToOne attribute as an @EmbeddedId in JPA?
问题描述
我正在询问并回答我自己的问题，但我不认为我的回答是最好的.如果您有更好的选择，请发布！
I'm asking and answering my own question, but i'm not assuming i have the best answer. If you have a better one, please post it!
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我有一对具有简单聚合关系的类:一个实例的任何实例都拥有另一个实例的一定数量.拥有类具有其自己的某种主键，并且拥有类通过相应的外键与该类具有多对一的关系.我希望拥有的班级有一个包含该外键以及一些附加信息的主键.
I have a pair of classes which are in a simple aggregation relationship: any instance of one owns some number of instances of the other. The owning class has some sort of primary key of its own, and the owned class has a many-to-one to this class via a corresponding foreign key. I would like the owned class to have a primary key comprising that foreign key plus some additional information.
为了争辩，让我们使用那些常年使用的收藏夹Order和OrderLine.
For the sake of argument, let's use those perennial favourites, Order and OrderLine.
SQL看起来像这样:
The SQL looks something like this:
-- 'order' may have been a poor choice of name, given that it's an SQL keyword!
create table Order_ (
orderId integer primary key
);
create table OrderLine (
orderId integer not null references Order_,
lineNo integer not null,
primary key (orderId, lineNo)
);
我想使用JPA将其映射到Java中.订单微不足道，可以使用@IdClass处理OrderLine. 这是该代码-该代码相当传统，我希望您能原谅我的特质.
I would like to map this into Java using JPA. Order is trivial, and OrderLine can be handled with an @IdClass. Here's the code for that - the code is fairly conventional, and i hope you'll forgive my idiosyncrasies.
但是，使用@IdClass涉及编写一个ID类，该类复制OrderLine中的字段.我想避免重复，所以我想改用@EmbeddedId.
However, using @IdClass involves writing an ID class which duplicates the fields in the OrderLine. I would like to avoid that duplication, so i would like to use @EmbeddedId instead.
但是，天真的尝试失败:
However, a naive attempt to do this fails:
@Embeddable
public class OrderLineKey {
@ManyToOne
private Order order;
private int lineNo;
}
OpenJPA拒绝将其用作@EmbeddedId.我没有尝试过其他提供程序，但是我不希望它们成功，因为JPA规范要求组成ID类的字段是基本的，而不是关系.
OpenJPA rejects the use of that as an @EmbeddedId. I haven't tried other providers, but i wouldn't expect them to succeed, because the JPA specification requires that the fields making up an ID class be basic, not relationships.
那我该怎么办?我该如何编写一个其键包含@ManyToOne关系但作为@EmbeddedId处理的类?
So, what can i do? How can i write a class whose key contains @ManyToOne relationship, but is handled as an @EmbeddedId?
推荐答案
我不知道一种不涉及重复任何字段的方法(抱歉！).但这可以通过直接复制关系字段的简单而标准的方式来完成.关键是JPA 2中引入的 @MapsId 批注.
I don't know of a way to do this which doesn't involve duplicating any fields (sorry!). But it can be done in a straightforward and standard way that involves duplicating only the relationship fields. The key is the @MapsId annotation introduced in JPA 2.
可嵌入的密钥类如下:
@Embeddable
public class OrderLineKey {
private int orderId;
private int lineNo;
}
嵌入实体类如下:
@Entity
public class OrderLine{
@EmbeddedId
private OrderLineKey id;
@ManyToOne
@MapsId("orderId")
private Order order;
}
@MapsId批注声明，对其有效地应用到的关系字段从嵌入的ID重新映射了一个基本字段.
The @MapsId annotation declares that the relationship field to which it is applied effectively re-maps a basic field from the embedded ID.
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