Mybatis中“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！