java - JPA 添加子记录 - 在父记录中有 3 个副本

Question

我有新闻实体和评论实体。我想创建有 0 条评论的新闻，然后向该新闻添加一些评论。

在数据库中，我有用于新闻信息的表 NEWS 和 COMMENTS 表中的 news_id 列。

新闻.java:

@Entity
@Table(name = "NEWS")
public class News implements Serializable{
    /**
     * serialVersionUID
     */
    private static final long serialVersionUID = 883279937885116359L;
    /**
     * News id
     */
    @Id
    @GeneratedValue(generator = "seq")
    @SequenceGenerator(name="seq", sequenceName="NEWS_SEQ",allocationSize=1)
    @Column(name = "NEWS_ID", nullable = false, unique = true)
    private Long id;
    /**
     * News short text
     */
    @Column(name = "SHORT_TEXT")
    private String shortText;
    /**
     * News full text
     */
    @Column(name = "FULL_TEXT")
    private String fullText;
    /**
     * News title
     */
    @Column(name = "TITLE")
    private String title;
    /**
     * News creation date
     */
    @Column(name = "CREATION_DATE")
    private Date creationDate;
    /**
     * News modification date
     */
    /**
     * News comments
     */
    @OneToMany(cascade = CascadeType.ALL, orphanRemoval= true, fetch = FetchType.EAGER)
    @JoinColumn(name = "NEWS_ID")
    private List<Comment> comments;

评论.java:

@Entity
@Table(name = "COMMENTS")
public class Comment implements Serializable{
    /**
     * serialVersionUID
     */
    private static final long serialVersionUID = -5697896094322498108L;
    /**
     * Comment id
     */
    @Id
    @GeneratedValue(generator = "seq")
    @SequenceGenerator(name="seq", sequenceName="COMMENTS_SEQ",allocationSize=1)
    @Column(name = "COMMENT_ID", nullable = false, unique = true)
    private Long id;
    /**
     * Comment text
     */
    @Column(name = "COMMENT_TEXT")
    private String commentText;
    /**
     * Comment creation date
     */
    @Column(name = "CREATION_DATE")
    private Date creationDate;
    /**
     * Id of the news which the comment is added to
     */
    @Column(name = "NEWS_ID")
    private Long newsId;

我有这样的代码:

            News news = new News();

            news.setTitle("qwerty");
            news.setShortText("qwerty");
            news.setFullText("qwerty");

            jpaNewsDAO.add(news);
            news = jpaNewsDAO.findById(news.getId());//everything is fine here
                                                     // 0 comments
            Comment comment = new Comment(null,"qwerty",new Date(),news.getId());
            jpaCommentDAO.add(comment);
            news = jpaNewsDAO.findById(news.getId());

但是在那之后，news.comments 有 3 个项目，其中包含评论的副本(即使具有相同的 id)，我不明白为什么。 DB中只有一条这样的评论。

添加评论:

@Override
    public Long add(Comment entity) throws DAOException {
        EntityManager manager = null;
        EntityTransaction transaction;
        try {
            manager = managerFactory.createEntityManager();
            transaction = manager.getTransaction();
            transaction.begin();
            entity.setId(null);
            manager.persist(entity);
            manager.flush();
            transaction.commit();
        } catch (PersistenceException e) {
            throw new DAOException(e);
        } finally {
            closeManager(manager);
        }
        return entity.getId();
    }

添加新闻:

@Override
    public Long add(News entity) throws DAOException {
        EntityManager manager = null;
        EntityTransaction transaction;
        try {
            manager = managerFactory.createEntityManager();
            transaction = manager.getTransaction();
            transaction.begin();
            entity.setId(null);

            List<Comment> comments = new ArrayList<Comment>();
            if (entity.getComments()!= null) {
                for (Comment comment : entity.getComments()) {
                    comments.add(manager.find(Comment.class, comment.getId()));
                }
            }
            entity.setComments(comments);

            manager.persist(entity);//add
            manager.flush();
            transaction.commit();
        } catch (PersistenceException e) {
            throw new DAOException(e);
        } finally {
            closeManager(manager);
        }
        return entity.getId();
    }

那为什么我有评论副本？

Answer 1

该问题是由于新闻和评论之间的关系被标记为 EAGER 造成的。有关为什么会出现这种情况的更多信息，请参阅此处:

Hibernate Criteria returns children multiple times with FetchType.EAGER

您可以将关系标记为 LAZY，这样可以防止出现问题。通常，最佳实践是以这种方式定义关系，并通过查询提示或 JPA 2.1 的新“实体图”功能等方式根据给定用途的需要启用预先获取关系。

https://blogs.oracle.com/theaquarium/entry/jpa_2_1_entity_graphs

现在您只需在 DAO 中添加以下行即可:

news.getComments().size()。

这将强制加载惰性集合。然后它显然将在您的网络层中可用。