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我在尝试更新实体时遇到以下错误。这个问题的解决方案是在持久化所有者之前必须持久化“拥有”实体，但我这样做了。或者用级联标记两个实体。

错误（我猜不需要完整的堆栈跟踪）：

2015-08-10T20:45:11.481+0200|Warning: A system exception occurred during an invocation on EJB ThreadLookUpService, method: public void main.java.services.ThreadLookUpService.setLastPost(long,main.java.entities.Post)
Caused by: java.lang.IllegalStateException: During synchronization a new object was found through a relationship that was not marked cascade PERSIST: main.java.entities.Post@6724f919.

（如果我添加cascade = CascadeType.ALL我得到另一个错误，告诉我外键不存在，但我只是坚持它。据我了解，如果我以正确的顺序坚持实体，我什至不必使用级联。

Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`forumcs`.`thethreads`, CONSTRAINT `fk_thethreads_posts1` FOREIGN KEY (`last_post`) REFERENCES `posts` (`idpost`) ON DELETE NO ACTION ON UPDATE NO ACTION)

)

在这里，我坚持 post(owned) 然后 Thethread(owner)：

post = postLookup.createPostAndGiveItBack(post);
threadLookup.setLastPost(thread.getThread().getIdthread(), post); //this gives error.

这是我保留帖子的方法，我使用了flush，因为我认为没有生成帖子ID或smtg：

@Override
public Post createPostAndGiveItBack(Post post) {
    em.persist(post);
    em.flush();
    return post;
}

这是我更新 Thethread 的 lastPost 的方法。我可以将它直接与我之前拉出的那个合并，但如果我这样做了，那么如果 2 个不同的用户观看同一页面并更新线程，我的 upvote 和 downvote 字段将被搞砸：

@Override
public void setLastPost(long id, Post post) {
    Query query = em.createQuery(FIND_THREAD_BY_ID);
    query.setParameter("id", id);
    Thethread thread = (Thethread) query.getSingleResult();
    thread.setLastPost(post);
}

发布实体：

@Entity
@Table(name = "posts")
@NamedQuery(name = "Post.findAll", query = "SELECT p FROM Post p")
public class Post implements Serializable, Comparable<Post> {
    private static final long serialVersionUID = 1L;

    @Id
    private int idpost;

    private String content;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "date_post")
    private Date datePost;

    private int downvotes;

    private int upvotes;

    // bi-directional many-to-one association to PostVote
    @OneToMany(mappedBy = "post")
    private List<PostVote> postVotes;

    // bi-directional many-to-one association to Post
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "reply_to_post")
    private Post replyTo;

    // bi-directional many-to-one association to Post
    @OneToMany(mappedBy = "replyTo")
    private List<Post> replies;

    // bi-directional many-to-one association to Thethread
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "threads_idthread")
    private Thethread thethread;

    // bi-directional many-to-one association to User
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "users_username")
    private User user;

    @Transient
    private double confidenceScore;

    public Post() {
    }

线程实体：

@Entity
@Table(name = "thethreads")
@NamedQuery(name = "Thethread.findAll", query = "SELECT t FROM Thethread t")
public class Thethread implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    private int idthread;

    private String content;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "date_posted")
    private Date datePosted;

    // I could get this by scanning the Threadvotes and count the number of downvotes
   // But I thought it was better like this for ease and performance.
    private int downvotes;

    @Column(name = "hot_score")
    private int hotScore;

    @Column(name = "is_pol")
    private String isPol;

    private String title;

    private String type;

    private int upvotes;

    // bi-directional many-to-one association to Post
    @OneToMany(mappedBy = "thethread")
    private List<Post> posts;

    // bi-directional many-to-one association to Category
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "categories_idcategory")
    private Category category;

    // UNI-directional many-to-one association to Post
    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "last_post")
    private Post lastPost;

    // bi-directional many-to-one association to User
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "posted_by")
    private User user;

    // bi-directional many-to-one association to ThreadVote
    @OneToMany(mappedBy = "thethread")
    private List<ThreadVote> threadVotes;
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1 回答 1

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我将此添加到我的实体 ID：

@GeneratedValue(strategy = GenerationType.IDENTITY)

它有效。

于 2015-08-10T23:00:21.770 回答