在Mybatis中association标签多层嵌套的问题

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

<resultMap id="BaseResultMap" type="a.b.c.d.e">
    <id column="id" property="id" />
    <result property="workTime" column="work_time" />
    <result property="model" column="model" />
    <result property="status" column="status" />
    <association property="interfaceUpstream" javaType="interfaceUpstream" columnPrefix="ui_">
      <id column="id" property="id" />
      <result property="interfaceName" column="interface_name" />
      <result property="interfaceType" column="interface_type" />
      <result property="frequency" column="frequency" />
      <result property="address" column="address" />
      <result property="templateOrSql" column="template_or_sql" />
      <result property="status" column="status" />
      <association property="systemInfo" javaType="SystemInfo" columnPrefix="sys_">
        <id column="id" property="id"/>
        <result property="systemName" column="system_name"/>
        <result property="systemNameEN" column="system_name_en"/>
        <result property="belong" column="belong"/>
        <result property="status" column="status"/>
      </association>
      <association property="serverInfo" javaType="ServerInfo" columnPrefix="ser_">
        <id column="id" property="id"/>
        <result property="ftpIp" column="ftp_ip"/>
        <result property="ftpPort" column="ftp_port"/>
        <result property="ftpAccount" column="ftp_account"/>
        <result property="ftpPassword" column="ftp_password"/>
      </association>
    </association>
  </resultMap>

<sql id="base_select">
    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
  </sql>

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：

在Mybatis中association标签多层嵌套的问题

这个返回集合有什么用呢

举个例子三张表

hr_job_department

在Mybatis中association标签多层嵌套的问题

hr_job_position

在Mybatis中association标签多层嵌套的问题

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

在Mybatis中association标签多层嵌套的问题

但是查询的时候，希望表示下面这样的结果

在Mybatis中association标签多层嵌套的问题

所以返回值是不止一个对象，这样就用到了集合嵌套

<resultMap id="userInfoMap" type="com.advancedc.hrsys.entity.UserInfo">
		<id column="id" property="id" />
		<result column="name" property="name" />
		<result column="gender" property="gender" />
		<result column="id_card" property="idCard" />
		<result column="is_married" property="isMarried" />
		<result column="phone" property="phone" />
		<result column="priority" property="priority" />
		<result column="entry_time" property="entryTime" />
		<result column="full_time" property="fullTime" />
		<result column="created_time" property="createdTime" />
		<result column="edited_time" property="editedTime" />
		<association property="jobDepartment" column="id"
			javaType="com.advancedc.hrsys.entity.JobDepartment">
			<id column="jdid" property="id" />
			<result column="jdname" property="name" />
		</association>
		<association property="jobPosition" column="id"
			javaType="com.advancedc.hrsys.entity.JobPosition">
			<id column="jpid" property="id" />
			<result column="jpname" property="name" />
		</association>
	</resultMap>

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 id="queryUserInfoBySomeone" resultMap="userInfoMap" resultType="com.advancedc.hrsys.entity.UserInfo">
		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
		<where>
			<if test="someone.id>0">
			and ui.id = #{someone.id}
			</if>
			<if test="someone.gender!=null">
			and ui.gender = #{someone.gender}
			</if>
			<if test="someone.name!=null">
			and ui.name = #{someone.name}
			</if>
			<if test="someone.idCard!=null">
			and ui.id_card = #{someone.idCard}
			</if>
			<if test="someone.isMarried!=null">
			and ui.is_married = #{someone.isMarried}
			</if>
			<if test="someone.jobDepartment!=null and someone.jobDepartment.id!=null">
			and ui.department_id = #{someone.jobDepartment.id}
			</if>
			<if test="someone.jobPosition!=null and someone.jobPosition.id!=null">
			and ui.position_id = #{someone.jonPosition.id}
			</if>
			<if test="someone.phone!=null">
			and ui.phone = #{someone.phone}
			</if>
			<if test="someone.entryTime!=null">
			and ui.entry_time = #{someone.entryTime}
			</if>
			<if test="someone.fullTime!=null">
			and ui.full_time = #{someone.fullTime}
			</if>
		</where>
	</select>

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;

以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。

原文地址：https://blog.csdn.net/weixin_42776772/article/details/104724922