这里先总结一下 association 存在的问题。

一、内嵌查询时存在报错Id找不到及内存溢出隐患
二、一对多关系数据重复问题
三、多层嵌套内层 association 查询结果为null 或 非预期的值

一、内嵌查询时存在报错Id找不到及内存溢出隐患

参考： https://www.lmlphp.com/user/57996/article/item/1356741/
错误示例：

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.lizhou.dao.StudentDao"><resultMap type="Student" id="studentResult"><id property="id" column="id"/><association property="grade" column="gradeId" select="com.lizhou.dao.GradeDao.getGrade"></association></resultMap><select id="getStudent" resultMap="studentResult">SELECT * FROM student<where><if test="id != null">AND id=#{id}</if></where></select></mapper> 

报错：Caused by: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named ‘id’ in ‘class java.lang.Integer’

正确示例：

<mapper namespace="com.lizhou.dao.GradeDao"><resultMap type="Grade" id="gradeResult"><id property="id" column="id"/></resultMap><select id="getGrade" parameterType="Grade" resultMap="gradeResult">SELECT * FROM grade WHERE id=#{id}</select></mapper> 

内存溢出隐患示例

错误示例：
StudentMapper.xml：

<mapper namespace="com.lizhou.dao.StudentDao"><resultMap type="Student" id="studentResult"><id property="id" column="id"/><association property="grade" column="gradeId" select="com.lizhou.dao.GradeDao.getGrade"></association></resultMap><select id="getStudent" resultMap="studentResult">SELECT * FROM student WHERE gradeId=#{gradeId}</select></mapper> 

GradeMapper.xml：

<mapper namespace="com.lizhou.dao.GradeDao"><resultMap type="Grade" id="gradeResult"><id property="id" column="id"/><collection property="studentList" column="id" ofType="Student" select="com.lizhou.dao.StudentDao.getStudent"></collection></resultMap><select id="getGrade" parameterType="Grade" resultMap="gradeResult">SELECT * FROM grade WHERE id=#{id}</select></mapper> 

解决方式：
分离写，分段查询

二、一对多关系数据重复问题

参考：https://blog.csdn.net/wuyezhiyu/article/details/81364974

 <resultMap id="commissionRec" type="CommissionRec" ><result property="sourceName" column="cmr_sourceName"/><result property="totalMoney" column="cmr_totalMoney"/><result property="totalCard" column="cmr_totalCard"/><result property="totalCommission" column="cmr_totalCommission"/><result property="commissionTimes" column="cmr_commissionTimes"/><result property="isProvided" column="cmr_isProvided"/><result property="userInfo.userId" column="us_userId"/><result property="userInfo.userName" column="us_userName"/><association property="memberCard" resultMap="memberCardTemplate" /> <!-- <association property="userInfo" resultMap="userInfo" /> --></resultMap>

 <resultMap id="userInfo" type="cn.sstech.member.marketing.model.base.BaseUserInfo" ><id property="userId" column="us_userId"/><result property="userName" column="us_userName"/></resultMap>

如上图所示查询结果中，第一条和第二条只有us_userId是不重复的，而剩下的字段都被注入到CommissionRec对象中。这时

1.用association注入userInfo mybatis会因为数据一样而将前两条数据合为一个CommissionRec对象，us_userId则随便调一个注入到userInfo对象中

2.用 <result property=“userInfo.userId” column=“us_userId”/> 这种制定具体字段名的方式注入，mybatis会因为记录数据有所不同而将前两条数据设为2个CommissionRec对象.

三、多层嵌套内层 association 查询结果为null 或 非预期

非预期

本人遇到的就是这个问题
以组织机构树的单张表为例，一般存的有ID及pre_id父级ID，当内嵌查询时，由于父级ID存在多条记录，导致查询结果并非预期

多层嵌套查询结果为null

参考：https://www.zhuxianfei.com/java/57394.html
错误示例：

<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">SELECTii.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>

结果： 内层的association查询的结果一直为null

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

正确示例：

SELECTii.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

总结

1、能不用就别用
2、如果用，请务必指定对应的字段或id
3、多层嵌套时请注意前缀的使用
4、内嵌sql避免递归