如何维护一个Map在 JPA2 中？

发布于 2024-10-11 03:42:41 字数 1056 浏览 1 评论 0原文

我有三个实体类（A、B、C）。 A 应包含将 B 实例映射到 C 实例的映射。 C 的所有实例均由某个 A 的映射拥有。我已经尝试过这个（省略 getters 和 setters）：

@Entity public class A {
  @Id @GeneratedValue private Long id;

  // I want to map this in an elegant way:
  @OneToMany(mappedBy="a") @MapKey(name="b") private Map<B, C> map;
}

@Entity public class B {
  @Id @GeneratedValue private Long id;
}

@Entity public class C {
  @Id @GeneratedValue private Long id;

  // I don't really want the following in Java, unidirectional access from A to C would suffice:
  @ManyToOne private A a;
  @ManyToOne private B b;
  // Can I get rid of a and b?
}

这提供了一个很好的模式（正是我想要的模式！），但是在 Java 中，由于现在有两种方法来指定关系：

a.map.put(b, c);

？

c.a = a;
c.b = b;

如果我只改变关联的一半会发生什么这看起来有问题。解决这个问题的最佳方法是什么？对于所需的单向访问，是否有一些更优雅的解决方案？

我考虑过将 C @Embeddable 并为 A.map 使用 @ElementCollection，但实际上 C 是不同实体的抽象基类，意味着它不能被@Embeddable。

原文

I have three entity classes (A, B, C). A should contain a map which maps instances of B to instances of C. All instances of C are owned by a map of some A. I've tried this (getters and setters omitted):

@Entity public class A {
  @Id @GeneratedValue private Long id;

  // I want to map this in an elegant way:
  @OneToMany(mappedBy="a") @MapKey(name="b") private Map<B, C> map;
}

@Entity public class B {
  @Id @GeneratedValue private Long id;
}

@Entity public class C {
  @Id @GeneratedValue private Long id;

  // I don't really want the following in Java, unidirectional access from A to C would suffice:
  @ManyToOne private A a;
  @ManyToOne private B b;
  // Can I get rid of a and b?
}

That gives a nice schema (exactly the one that I want!), but in Java there's ugly duplication given that there are now two ways to specify the relation:

a.map.put(b, c);

and

c.a = a;
c.b = b;

What happens if I just change one half of the association? That looks problematic. What is the best way around that? Isn't there some more elegant solution for the required unidirectional access?

I considered making C @Embeddable and use an @ElementCollection for A.map, but in reality C is an abstract base class for different entities which means that it can't be made @Embeddable.

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