mybatis Result Maps对结果分组2--多层一对多

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在之前的文章中说的是一个一对多情况下mybatis的自动分组，这篇说下多个一对多的情况。

比如qq空间里的说说



可以看到，说说和评论是一对多，评论又和回复是一对多。

mood

public class Mood {
private int mood_id;
private String mood_content;
private String mood_time;
private User user;
private List<MoodComment> moodComments;
//getter,setter..
}

mood comment

public class MoodComment {
private int moodcommentid;
private String comment_content;
private String comment_time;
private User user;
private Mood mood;
private List<MoodCommentReply> moodCommentReplys;
//getter,setter
}

mood reply

public class MoodCommentReply {
private int moodreplyid;
private String reply_content;
private String reply_time;
private User user;
private MoodComment moodComment;
//getter,setter
}

mood表



moodcomment表



moodcommentreply表



user表



表数据

做法很容易想到，就是把上一篇resultmap改一下就可以了。

<resultMap id="MoodResult" type="Mood">
<id property="mood_id" column="mood_id" />
<association property="user" javaType="User">
<id property="id" column="mood_userid"/>
<result property="name" column="mood_user"/>
</association>
<collection property="moodComments" ofType="MoodComment">
<id property="moodcommentid" column="moodcommentid" />
<association property="user" javaType="User">
<id property="id" column="comment_userid" />
<result property="name" column="comment_user"/>
</association>
<collection property="moodCommentReplys" ofType="MoodCommentReply">
<association property="user" javaType="User">
<id property="id" column="reply_userid" />
<result property="name" column="reply_user"/>
</association>
</collection>
</collection>
</resultMap>

可以看到在<collection>里面再加一个<collection>.不过，要注意在里面的<collection>加上<id>,否则mybatis不会自动进行第二次分组。

至于sql，还是把三个表数据一并取出就可以了，mybatis会自动分组。

SELECT u1.name AS mood_user,u2.name AS comment_user,u3.name AS reply_user,
mood.mood_id,mood.id AS mood_userid,mood_content,mood_time,
moodcomment.moodcommentid,moodcomment.id AS comment_userid,moodcomment.comment_content,moodcomment.comment_time,
moodcommentreply.moodcommentid,moodcommentreply.id AS reply_userid,reply_content,reply_time FROM mood
LEFT JOIN moodcomment ON moodcomment.mood_id=mood.mood_id
LEFT JOIN moodcommentreply ON moodcommentreply.moodcommentid=moodcomment.moodcommentid
LEFT JOIN USER AS u1 ON mood.id=u1.id
LEFT JOIN USER AS u2 ON moodcomment.id=u2.id
LEFT JOIN USER AS u3 ON moodcommentreply.id=u3.id
ORDER BY mood_time DESC,moodcomment.comment_time DESC,reply_time DESC

问题有来了，如果只要评论的前2条，但是那2条评论的所有回复要全部选出。

比如



还是用(译)如何在sql中选取每一组的第一行/最后行/前几行里面提到的user variables方法。

SET @num := 0, @type := '';
SELECT u1.name AS mood_user,u2.name AS comment_user,u3.name AS reply_user,
mood.mood_id,mood.id AS mood_userid,mood_content,mood_time,
moodcomment2.moodcommentid,moodcomment2.id AS comment_userid,moodcomment2.comment_content,moodcomment2.comment_time,
moodcommentreply.moodcommentid,moodcommentreply.id AS reply_userid,reply_content,reply_time FROM mood
LEFT JOIN
(SELECT *
FROM (
SELECT moodcommentid,comment_content,comment_time,id,mood_id,
@num := IF(@type = mood_id, @num + 1, 1) AS row_number,
@type := mood_id AS dummy
FROM moodcomment
ORDER BY mood_id
) AS moodcomment1 WHERE moodcomment1.row_number <= 2) AS moodcomment2
ON moodcomment2.mood_id=mood.mood_id
LEFT JOIN moodcommentreply ON moodcommentreply.moodcommentid=moodcomment2.moodcommentid
LEFT JOIN USER AS u1 ON mood.id=u1.id
LEFT JOIN USER AS u2 ON moodcomment2.id=u2.id
LEFT JOIN USER AS u3 ON moodcommentreply.id=u3.id
ORDER BY mood_time DESC,moodcomment2.comment_time DESC,reply_time DESC

注意几个moodcomment的命名空间。还有需要在jdbcUrl中加上allowMultiQueries=true，因为set variable也是一次查询。