JPA表有2个主键字段 [英] JPA table with 2 primary key fields

问题描述

我有一个只包含2个字段的表。该表具有由这两个字段组成的复合PK。

I have a table which contains only 2 fields. The table has a composite PK formed by these two fields.

当使用Netbeans从数据库创建实体bean时，实体bean不会像其他具有更多数据的表一样自动创建超过2个字段。

When using Netbeans for creating entity beans from database, the entity bean is not created automatically like other tables that have more than 2 fields.

所以我想我需要自己创建实体bean。创建此实体bean的最佳做法是什么？它是否必须包含 COMPOSITE KEY 对象？

So I guess I need to create the entity bean myself. What is the best practice for creating this entity bean? Does it have to contain COMPOSITE KEY object or not?

推荐答案

我不要使用NetBeans，所以我无法真正说出它的映射工具。

I don't use NetBeans so I can't really say anything about its mapping tools.

对于映射复合键，有几个选项。你可以

For mapping a composite key, there are a couple of options. You can

• 定义一个单独的 @Embeddable 具有PK字段的对象并将其用作 @EmbeddedId code> @Entity class

• Define a separate @Embeddable object with the PK fields and use it as @EmbeddedId in your @Entity class

@Embeddable
public class MyCompositePK { 
    @Column
    private String fieldA;
    @Column
    private String fieldB;
}
@Entity 
public class MyBean { 
    @EmbeddedId
    private MyCompositePK id;
    @Column
    private String fieldC;
}

使用PK字段定义未映射的POJO并使用它如 @IdClass @Entity 中的。

@Entity
@IdClass(value=ClassAB.ClassABId.class)
public class ClassAB implements Serializable {
    private String idA;
    private String idB;

    @Id
    @Column(name="ID_A")
    public String getIdA(){ return idA; }
    public void setIdA(String idA){ this.idA = idA; }

    @Id
    @Column(name="ID_B")
    public String getIdB(){ return idB; }
    public void setIdB(String idB){ this.idB = idB; }

    static class ClassABId implements Serializable {
        private String idA;
        private String idB;

        public String getIdA(){ return idA; }
        public void setIdA(String idA){ this.idA = idA; }

        public String getIdB(){ return idB; }
        public void setIdB(String idB){ this.idB = idB; }

        // implement equals(), hashcode()
    }
}

在此示例中， ClassABId 是一个静态内部类，只是为了方便起见。

In this example ClassABId is a static inner class just for convenience.

Pascal Thivent对这个问题的出色回答也解释了这些选项：如何使用Hibernate映射复合密钥？。

These options are also explained in Pascal Thivent's excellent answer to this question: How to map a composite key with Hibernate?.

这个相关问题讨论了这些方法之间的差异：< a href =https://stackoverflow.com/q/212350/851811>我应该使用哪种anotation：@IdClass或@EmbeddedId 。请注意，字段的声明与 @IdClass 方法重复。

This related question discusses differences between these approaches: Which anotation should I use: @IdClass or @EmbeddedId. Notice the fields' declaration gets duplicated with the @IdClass approach.

无论如何，我认为没有其他方法可以创建两个类。这就是我提出这个问题的原因：映射一个只包含没有@IdClass或@EmbeddedId的复合PK的类。似乎有一个特定于hibernate的功能。

Anyhow, I don't think there's an alternative to creating two classes. That's why I asked this question : Mapping a class that consists only of a composite PK without @IdClass or @EmbeddedId. It seems there's an hibernate-specific feature for this.

作为旁注，如果你能控制数据库结构，你也可以考虑避免使用复合键。 有一些理由这样做。

As a side note, if you've got control over the DB structure, you might also consider avoiding composite keys. There are some reasons to do so.

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