問題描述

我正在使用帶有注釋的 Hibernate 3.5.2-FINAL 來指定我的持久性映射.我正在努力為應用程序和一組平臺之間的關系建模.每個應用程序都可用于一組平臺.

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.

從我所做的所有閱讀和搜索中，我認為我需要將平臺枚舉類作為實體持久化，并有一個連接表來表示多對多關系.我希望關系在對象級別是單向的，也就是說，我希望能夠獲取給定應用程序的平臺列表，但我不需要找出給定平臺的應用程序列表.

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.

這是我的簡化模型類:

@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...
}

當我運行 Hibernate hbm2ddl 工具時，我看到以下內容(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)
);

相應的外鍵也從該表創建到應用程序表和平臺表.到目前為止一切順利.

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

我遇到的一個問題是當我嘗試持久化應用程序對象時:

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 表中創建適當的行，即為 Windows 和 Linux 創建一行，如果它們不存在的話.然后，應該為新應用程序創建一行，然后在連接表中創建新應用程序與兩個平臺之間的映射.

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.

我遇到的一個問題是出現以下運行時異常:

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，當我嘗試持久化應用程序時，平臺集沒有被持久化.級聯注釋應該可以解決這個問題，但我不知道出了什么問題.

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.

所以我的問題是:

1. 有沒有更好的方法來模擬我想做的事情，例如使用 Enum 合適嗎?
2. 如果我的模型沒問題，如何正確持久化所有對象?

我已經為此苦苦掙扎了好幾個小時，并嘗試重新創建上面的所有代碼，但它可能不完整和/或不準確.我希望有人會指出一些明顯的事情！

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!

推薦答案

你應該決定你的Platform是否是一個entity.

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

如果是實體，則不能是enum，因為可能的平臺列表存儲在數據庫中，而不是應用程序中.它應該是帶有 @Entity 注釋的常規類，并且您將具有正常的多對多關系.

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.

如果不是實體，則不需要TBL_PLATFORM 表，也沒有多對多關系.在這種情況下，您可以將一組 Platform 表示為帶有位標志的整數字段，或者表示為簡單的一對多關系.JPA 2.0 使用 @ElementCollection 使后一種情況變得簡單:

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 沒有注釋.

這篇關于在多對多單向映射中持久化枚舉集的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！