问题描述

我正在使用带有注释的 Hibernate 3.5.2-FINAL 来指定我的持久性映射.我正在努力为应用程序和一组平台之间的关系建模.每个应用程序都可用于一组平台.

I'm using Hibernate 3.5.2-FINAL with annotations to specify my persistence mappings. I'm struggling with modelling a relationship between an Application and a set of Platforms. Each application is available for a set of platforms.

从我所做的所有阅读和搜索中，我认为我需要将平台枚举类作为实体持久化，并有一个连接表来表示多对多关系.我希望关系在对象级别是单向的，也就是说，我希望能够获取给定应用程序的平台列表，但我不需要找出给定平台的应用程序列表.

From all the reading and searching I've done, I think I need to have the platform enum class be persisted as an Entity, and to have a join table to represent the many-to-many relationship. I want the relationship to be unidirectional at the object level, that is, I want to be able to get the list of platforms for a given application, but I don't need to find out the list of applications for a given platform.

这是我的简化模型类:

@Entity
@Table(name = "TBL_PLATFORM")
public enum Platform {
    Windows,
    Mac,
    Linux,
    Other;

    @Id
    @GeneratedValue
    @Column(name = "ID")
    private Long id = null;

    @Column(name = "NAME")
    private String name;

    private DevicePlatform() {
        this.name = toString();
    }

    // Setters and getters for id and name...
}

@Entity
@Table(name = "TBL_APP")
public class Application extends AbstractEntity implements Serializable {
    private static final long serialVersionUID = 1L;

    @Column(name = "NAME")
    protected String _name;

    @ManyToMany(cascade = javax.persistence.CascadeType.ALL)
    @Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE})
    @JoinTable(name = "TBL_APP_PLATFORM", 
              joinColumns = @JoinColumn(name = "APP_ID"),
              inverseJoinColumns = @JoinColumn(name = "PLATFORM_ID"))
    @ElementCollection(targetClass=Platform.class)
    protected Set<Platform> _platforms;

    // Setters and getters...
}

当我运行 Hibernate hbm2ddl 工具时，我看到以下内容(我使用的是 MySQL):

When I run the Hibernate hbm2ddl tool, I see the following (I'm using MySQL):

create table TBL_APP_PLATFORM (
    APP_ID bigint not null,
    PLATFORM_ID bigint not null,
    primary key (APP_ID, PLATFORM_ID)
);

相应的外键也从该表创建到应用程序表和平台表.到目前为止一切顺利.

The appropriate foreign keys are also created from this table to the application table and platform table. So far so good.

我遇到的一个问题是当我尝试持久化应用程序对象时:

One problem I'm running into is when I try to persist an application object:

Application newApp = new Application();
newApp.setName("The Test Application");
Set<DevicePlatform> platforms = EnumSet.of(Platform.Windows, Platform.Linux);
newApp.setPlatforms(platforms);
applicationDao.addApplication(newApp);

我希望在 Platform 表中创建适当的行，即为 Windows 和 Linux 创建一行，如果它们不存在的话.然后，应该为新应用程序创建一行，然后在连接表中创建新应用程序与两个平台之间的映射.

What I would like to happen is for the appropriate rows in the Platform table to created, i.e. create a row for Windows and Linux, if they don't already exist. Then, a row for the new application should be created, and then the mapping between the new application and the two platforms in the join table.

我遇到的一个问题是出现以下运行时异常:

One issue I'm running into is getting the following runtime exception:

2010-06-30 13:18:09,382 6613126-0 ERROR FlushingEventListener Could not synchronize database state with session org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.example.model.Platform

不知何故，当我尝试持久化应用程序时，平台集没有被持久化.级联注释应该可以解决这个问题，但我不知道出了什么问题.

Somehow, the platform set is not being persisted when I try to persist the application. The cascade annotations are supposed to take care of that, but I don't know what's wrong.

所以我的问题是:

1. 有没有更好的方法来模拟我想做的事情，例如使用 Enum 合适吗?
2. 如果我的模型没问题，如何正确持久化所有对象?

我已经为此苦苦挣扎了好几个小时，并尝试重新创建上面的所有代码，但它可能不完整和/或不准确.我希望有人会指出一些明显的事情！

I've been struggling with this for hours, and I've tried to recreate all of the code above, but it might not be complete and/or accurate. I'm hoping someone will point out something obvious!

推荐答案

你应该决定你的Platform是否是一个entity.

You should decide whether your Platform is an entity or not.

如果是实体，则不能是enum，因为可能的平台列表存储在数据库中，而不是应用程序中.它应该是带有 @Entity 注释的常规类，并且您将具有正常的多对多关系.

If it's an entity, it can't be an enum, because list of possible platforms is stored in the database, not in the application. It should be a regular class with @Entity annotation and you will have a normal many-to-many relation.

如果不是实体，则不需要TBL_PLATFORM 表，也没有多对多关系.在这种情况下，您可以将一组 Platform 表示为带有位标志的整数字段，或者表示为简单的一对多关系.JPA 2.0 使用 @ElementCollection 使后一种情况变得简单:

If it isn't an entity, then you don't need TBL_PLATFORM table, and you don't have a many-to-many relation. In this case you can represent a set of Platforms either as an integer field with bit flags, or as a simple one-to-many relation. JPA 2.0 makes the latter case simple with @ElementCollection:

@ElementCollection(targetClass = Platform.class) 
@CollectionTable(name = "TBL_APP_PLATFORM",
    joinColumns = @JoinColumn(name = "APP_ID"))
@Column(name = "PLATFORM_ID")
protected Set<Platform> _platforms; 

-

create table TBL_APP_PLATFORM (   
    APP_ID bigint not null,   
    PLATFORM_ID bigint not null, -- the ordinal number of enum value   
    primary key (APP_ID, PLATFORM_ID)   
);

和 enum Platform 没有注释.

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