java - 使用标准(Hibernate)仅获取最后的条目

Question

我有两个实体 Issue和 Issue_Tracker .

问题.java:

public class Issue implements Serializable
        {
        private Integer issue_id;
        private String  issue_description;
        private Date issue_raised_date;
        private Set<Issue_Tracker> issueTracker = new HashSet<Issue_Tracker>(0);



        @OneToMany(fetch=FetchType.LAZY, mappedBy="issue_id")
        public Set<Issue_Tracker> getIssueTracker() {
        return issueTracker;
    }

        public void setIssueTracker(Set<Issue_Tracker> issueTracker) {
        this.issueTracker = issueTracker;

Issue_Tracker.java:

public class Issue_Tracker implements Serializable
{  
     private Integer   issue_id;
     private String    tracker_status;
     private Timestamp tracked_time;

当我离开加入两个表时，我使用查询获得如下数据:

SELECT issues.issue_id,
       issues.issue_description,
       issue_tracker.tracker_status,
       issue_tracker.tracked_time
FROM issues
     LEFT JOIN issue_tracker on issues.issue_id = issue_tracker.issue_id 
     WHERE issues.status = "Escalate To"

输出为

  issue_id  tracker_status      tracked_time
    123     Assigned           1/8/2013 11:44   //1st entry
    123     Assigned           1/9/2013 11:45
    123     Completed          1/10/2013 12:52  // last entry

    32      Assigned           1/9/2013 16:46   //1st entry

    32      Assigned           1/10/2013 18:46  //last entry

    33      Assigned           1/9/2013 16:47   //1st entry

    33      Cancel             1/9/2013 17:49  //last entry

现在当我使用标准进行这些连接时，我得到了这些结果

      issue_id  tracker_status      tracked_time
        123     Assigned           1/8/2013 11:44 //1st entry of issue_id 123
        32      Assigned           1/9/2013 16:46
        33      Assigned           1/9/2013 16:47

使用以下标准

         Criteria criteria = session.createCriteria(Issue.class);
         criteria.setFirstResult(from);
         criteria.setMaxResults(size);
         criteria.setFetchMode("Issue.issueTracker",FetchMode.JOIN);
         criteria.add(Restrictions.eq("status", "Escalate To"));

现在预期的输出是

      issue_id  tracker_status      tracked_time
        123     Completed          1/10/2013 12:52 // last entry
        32      Assigned           1/10/2013 18:46 //last entry
        33      Cancel             1/9/2013 17:49  //last entry

如何实现这一点，自从过去三天以来我一直被困在这里，任何帮助都会很棒，是否因为我正在尝试一对多映射的这条线而发生

private Set<Issue_Tracker> issueTracker = new HashSet<Issue_Tracker>(0);

因为 Set 不允许重复值，如果是这样，可能的方法是什么。

Answer 1

试试这个:

SELECT i.issue_id, i.issue_description,
       it.tracker_status, it.tracked_time
FROM issues i 
LEFT JOIN ( SELECT it.issue_id, it.tracker_status, it.tracked_time 
            FROM issue_tracker it 
            INNER JOIN (SELECT issue_id, MAX(tracked_time) tracked_time 
                        FROM issue_tracker GROUP BY issue_id
                       ) A ON it.issue_id = A.issue_id AND it.tracked_time = A.tracked_time 
          ) it ON i.issue_id = it.issue_id 
WHERE i.status = "Escalate To";