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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
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。
