Mybatis中association标签多层嵌套问题怎么解决

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association标签多层嵌套问题

mybatis里查询使用嵌套association标签时，发现内层的association查询的结果一直为null

排查

• 检查sql执行情况，发现有数据返回，排除

• 检查property的值是否和pojo中的对应，值一致，排除

• 检查column的值是否和数据库的相对应，相对应，排除

那么应该是mybatis没有把数据映射到位了，经过排查是association中columnPrefix被不对应

    SELECT    ii.Id,    ii.model,    ii.status,    ii.work_time,    ui.id AS ui_id,    ui.interface_name AS ui_interface_name,    ui.interface_type AS ui_interface_type,    ui.frequency AS ui_frequency,    ui.address AS ui_address,    ui.template_or_sql AS ui_template_or_sql,    ui.status AS ui_status,    sys.id AS sys_id,    sys.system_name AS sys_system_name,    sys.system_name_en AS sys_system_name_en,    sys.belong AS sys_belong,    sys.status AS sys_status,    ser.id AS ser_id,    ser.ftp_ip AS ser_ftp_ip,    ser.ftp_port AS ser_ftp_port,    ser.ftp_account AS ser_ftp_account,    ser.ftp_password AS ser_ftp_password  

从代码上看没有什么问题

原因是association在进行多层嵌套时，mybatis会将外层association的columnPrefix值与内层的进行并合，

如外层columnPrefix值位ui_, 内层为sys_， 那么在SQL中就不能这样 sys.id AS sys_id 了，需要将ui_前缀加上，变成 sys.id AS ui_sys_id ,这样mybatis在匹配的时候才会将数据映射到对应association上

正常代码如下

SELECT    ii.Id,    ii.model,    ii.status,    ii.work_time,    ui.id AS ui_id,    ui.interface_name AS ui_interface_name,    ui.interface_type AS ui_interface_type,    ui.frequency AS ui_frequency,    ui.address AS ui_address,    ui.template_or_sql AS ui_template_or_sql,    ui.status AS ui_status,    sys.id AS ui_sys_id,    sys.system_name AS ui_sys_system_name,    sys.system_name_en AS ui_sys_system_name_en,    sys.belong AS ui_sys_belong,    sys.status AS ui_sys_status,    ser.id AS ui_ser_id,    ser.ftp_ip AS ui_ser_ftp_ip,    ser.ftp_port AS ui_ser_ftp_port,    ser.ftp_account AS ui_ser_ftp_account,    ser.ftp_password AS ui_ser_ftp_password

问题解决！

association集合嵌套

学了一下mybatis的查询返回值的集合嵌套，先查了查官网：

这个返回集合有什么用呢

举个例子三张表

hr_job_department

hr_job_position

第三张表里在表示部门和职位的时候只用了上面两张表的主键

但是查询的时候，希望表示下面这样的结果

所以返回值是不止一个对象，这样就用到了集合嵌套

只需要知道：

（1）column表示数据库字段

（2）property表示Java里的值

而且我这里的主键都是id所以会出现重名的情况，在SQL语句里，查询时就要赋予别名才能加以区分，返回结果resultMap就如上图所示

                SELECT                ui.id,                ui.name,                ui.gender,                ui.id_card,                ui.is_married,                ui.department_id,                ui.position_id,                ui.phone,                ui.priority,                ui.entry_time,                ui.full_time,                ui.created_time,                ui.edited_time,                jd.id jdid,                jd.name jdname,                jp.id jpid,                jp.name jpname                FROM                hr_user_info ui                INNER JOIN                hr_job_department jd                ON                ui.department_id=jd.id                INNER JOIN                hr_job_position jp                ON                ui.position_id=jp.id                                                                and ui.id = #{someone.id}                                                                        and ui.gender = #{someone.gender}                                                                        and ui.name = #{someone.name}                                                                        and ui.id_card = #{someone.idCard}                                                                        and ui.is_married = #{someone.isMarried}                                                                        and ui.department_id = #{someone.jobDepartment.id}                                                                        and ui.position_id = #{someone.jonPosition.id}                                                                        and ui.phone = #{someone.phone}                                                                        and ui.entry_time = #{someone.entryTime}                                                                        and ui.full_time = #{someone.fullTime}                                                

上图用了INNER JOIN来查询看上去挺简洁的，有一种不简洁的写法如下，虽然也能得到结果，但是不知道性能对比如何

SELECT                ui.id,                ui.name,                ui.gender,                ui.id_card,                ui.is_married,                ui.department_id,                ui.position_id,                ui.phone,                ui.priority,                ui.entry_time,                ui.full_time,                ui.created_time,                ui.edited_time,                (select id jdid from hr_job_department jd where jd.id=ui.department_id) jdid,                (select name jdname from hr_job_department jd where jd.id=ui.department_id) jdname,                (select id jpid from hr_job_position jp where jp.id=ui.position_id) jpid,                (select name jpname from hr_job_position jp where jp.id=ui.position_id) jpname                FROM                hr_user_info ui;

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