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java - hibernate @CollectionOfElements 和@Parent

转载 作者:行者123 更新时间:2023-11-29 09:23:38 27 4
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我完成了以下映射:

@Entity
@Table(name = "NWS_NEWS")
public class News implements Serializable {

private static final long serialVersionUID = 5246618151933389186L;

private String id;
private List<Picture> pictures;

+ OTHER fields / getters / setters, no matter

@Id
@GeneratedValue(generator = "juuid")
@Column(length = 36)
public String getId() {
return id;
}


@CollectionOfElements
@JoinTable(name = "NWS_PICTURES",joinColumns = @JoinColumn(name="NEWS_ID"))
@CollectionId(
columns= @Column(name="PICTURE_ID"),
type=@Type(type="long"),
generator="sequence")
public List<Picture> getPictures() {
return pictures;
}

public void setPictures(List<Picture> pictures) {
this.pictures = pictures;
}



}

我的照片是:

@Embeddable
public class Picture implements Serializable {

private static final long serialVersionUID = -1397366206984323622L;

private News news;

private String path;

private ImageSize imageSize;


@Parent
public News getNews() {
return this.news;
}

@Column(name = "path", nullable=false)
public String getPath() {
return path;
}

@Enumerated(EnumType.STRING)
@Column(name = "size", nullable=false)
public ImageSize getImageSize() {
return imageSize;
}


public void setImageSize(ImageSize imageSize) {
this.imageSize = imageSize;
}
public void setNews(News news) {
this.news = news;
}
public void setPath(String path) {
this.path = path;
}


}

我的 dao 测试是:

@Test
public void testAddPicturesToNews() {

News newsToSave = new News();
// Create big picture
Picture pBig = new Picture();
pBig.setImageSize(ImageSize.BIG);
pBig.setPath("/tmp/blabla_big.jpg");
// Create medium picture
Picture pMedium = new Picture();
pMedium.setImageSize(ImageSize.MEDIUM);
pMedium.setPath("/tmp/blabla_med.jpg");
// Set the pictures in the news
List<Picture> picturesList = new ArrayList<Picture>();
picturesList.add(pBig);
picturesList.add(pMedium);
newsToSave.setPictures(picturesList);
// Save the news
this.newsDAO.saveOrUpdate(newsToSave);
String newsId = newsToSave.getId();
News newsLoaded = this.newsDAO.findById(newsId);
List<Picture> picturesLoaded = newsLoaded.getPictures();
for ( Picture pictureLoaded : picturesLoaded ) {
System.out.println(pictureLoaded.getPath());
System.out.println(pictureLoaded.getImageSize());
System.out.println(pictureLoaded.getNews());
System.out.println("\n");
}

}

但是输出是:

/tmp/blabla_big.jpg
BIG
null


/tmp/blabla_med.jpg
MEDIUM
null

其实我不明白为什么 getNews() 在子实体实体中返回 null,而它有“@Parent”注解。难道我做错了什么?

无论如何,让父实体成为子实体的概念对我来说似乎有点奇怪,因为如果我这样做会发生什么:

News news1 = new News();
News news2 = new News();
List<Picture> picList = new ArrayList<Picture>();
Picture picture1 = new Picture();
picturesList.add(picture1);
picture1.setNews(news2);
news1.setPictures(picList);
this.newsDAO.saveOrUpdate(news1);
this.newsDAO.saveOrUpdate(news2);

如果同一张图片将出现在 news1 列表中,而且其父级设置为 news2,会发生什么情况???

我想我会没有那个 parent ,我不需要那么多,但这只是好奇......谢谢

顺便说一句,我希望每个新闻只有一张图片 -> 同一条新闻不能有 2 张小图片。那么是否可以在我的嵌入式实体中对 {news_id , imageSize} 添加唯一约束?我不知道该怎么做,因为在我的图片可嵌入实体中没有声明 id 字段

最佳答案

我不熟悉 @Embeddable 的 @Parent 注释,但对于“真实”关系,总是建议这样做:

// News class
public void setPictures(List<Picture> pictures) {
this.pictures = pictures;
for (Picture picture : pictures) {
picture.setNews(this);
}
}

public void addPicture(Picture picture) {
this.pictures.add(picture);
picture.setNews(this);
}

请记住,与关系模型相反,OOP 只有“单向”导航的概念,您应该自己构建“双向”导航。将此行为封装在 setter 中会使它对您的消费者透明。所以,我不确定为什么你的@Parent 不起作用,但我会尝试做相反的事情:

// what you have:
newsToSave.setPictures(picturesList);

// what I'd try:
pMedium.setNews(newsToSave);

What would happen since the same picture will be in news1 list, but also its parent was set to news2???

好吧,@Embeddable 是一个“嵌入”到另一个对象中的对象,这意味着它只属于另一个对象(父对象)。因此,它应该只包含一个父级。如果您更改父级,它将只属于这个新父级。如果您需要一个对象 (Picture) 与其他对象 (News) 建立关系,您将需要一个 @ManyToMany(如果另一个对象是 News ,也可能链接到几张图片)

关于java - hibernate @CollectionOfElements 和@Parent,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/4525789/

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