如何使用无状态会话从集合中删除元素？ （ConstraintViolationException）

Question

我有

@ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
public Linf getLinf() {
    return linf;
}

和

@OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, targetEntity = MessageEntry.class)
public Set<MessageEntry> getMessageEntries() {
    return messageEntries;
}

我需要从数据库中删除单个messageEntry。如果我说sess.delete（messageEntry），那么我在Linf.messageEntries集合中得到索引异常。无状态会话无法加载集合，所以我有 手动加载Linf.messageEntries的元素，然后删除其中一个元素：

            List linfs = sess.createQuery(
                    "SELECT l FROM Linf l " +
                            "JOIN l.messageEntries e WHERE e=:e")
                    .setParameter("e", messageEntry).list();

            if (linfs.size()>1) throw new RuntimeException();

            Linf linf = (Linf) linfs.get(0);

            List<MessageEntry> curEntries =
                    sess.createQuery(
                            "SELECT e FROM Linf l " +
                            "JOIN l.messageEntries e WHERE l=:l")
                            .setParameter("l", linf).list();

            for (int i = 0; i < curEntries.size(); i++) {
                if (curEntries.get(i).getId().equals(messageEntry.getId())) {
                    curEntries.remove(i);
                    break;
                }
            }

            Set<MessageEntry> cur = new HashSet<MessageEntry>();
            cur.addAll(curEntries);
            linf.setMessageEntries(cur);

            messageEntry.setLinf(null);
            sess.update(messageEntry);
            sess.update(linf);
            sess.delete(messageEntry);

我得到ConstraintViolationException：无法删除或更新父行：外键约束失败（db。linf_messageentry，CONSTRAINT FK5039A8B5E770809A FOREIGN KEY（messageEntries_id）REFERENCES { {1}}（messageentry））。我该如何执行此任务？谢谢。

Answer 1

您的映射是错误的。使用MessageEntry中的外键，不是使用双向一对多关联，而是使用连接表（linf_messageentry）进行OneToMany关联，使用外键进行另一个不同的ManyToOne关联。

必须使用mappedBy属性将一边标记为多边的反面：

// This is the owner side of the association, because it doesn't have 
// the mappedBy attribute.
// it uses a join column by default
@ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
public Linf getLinf() {
    return linf;
}

和

// This is the inverse side, because it has the mappedBy attribute
// since it's mappedBy linf, hibernate uses the same mapping as the one
// described on the linf property: a join column
@OneToMany(mappedBy = "linf", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
public Set<MessageEntry> getMessageEntries() {
    return messageEntries;
}

另外，请注意：

• targetEntity属性是完全冗余的：Hibernate从集合的类型中知道目标实体：Set<MessageEntry>
ManyLoOne上的
• cascade = CascadeType.ALL没有多大意义。如果删除MessageEntry，则不希望删除其Linf。