Mybatis中多表关联时，怎么利用association优雅写resultMap来映射vo
- 2019 年 11 月 14 日
- 笔记
前言
有好一阵没碰mybatis了，这次的项目基于性能考虑，选了mybatis，写着写着，发现有下面的需求，比如两表联查，取其中各一部分字段，怎么更方便地用vo来接，这里犯了难；
我想的是，因为这个sql联查的vo，能不能直接使用两个表的po来接呢，比如下面这种：
Users、SeatInformation都是对应了数据库两张表的po @Data public class UserSeatUnionQueryVO { private Users users; private SeatInformation seatInformation; }折腾了2个小时，网上各种找不到类似需求，终于搞出来了，还是值得记录和分享。
一、两表关联，映射到如下类型vo（拷贝单表po属性，组合另一单表的po），怎么写
sql：
<select id="selectOnlineUserBySeatState" resultMap="UserSeatUnionQueryVOResultMap"> SELECT u.`user_id`,u.`account_status`,s.* FROM users u, seat_information s WHERE u.`user_id` = s.`user_id` AND u.`account_status` = 1 AND u.`delete_status` = 1 AND u.`center_id` = 0 AND s.`token` IS NOT NULL AND s.`seat_state` = 1 AND s.delete_status = 1 </select>假设我想映射的vo如下：
@Data public class UserSeatUnionQueryVO { /** * 用户Id */ @ApiModelProperty(value = "用户Id") @TableId(value = "user_id", type = IdType.ID_WORKER) private Long userId; /** * 账号状态，-1停用，1启用 */ @ApiModelProperty(value = "账号状态，-1停用，1启用") private Integer accountStatus; /** * 这里的SeatInformation是表seat_information对应的po */ private SeatInformation seatInformation; }则mapper中应该这样写：
UsersMapper.xml <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.ceiec.cad.mapper.UsersMapper"> <!-- 通用查询映射结果 --> <resultMap id="BaseResultMap" type="com.ceiec.cad.model.Users"> <id column="user_id" property="userId" /> <result column="account_status" property="accountStatus" /> </resultMap> <resultMap id="UserSeatUnionQueryVOResultMap" type="com.ceiec.cad.service.UserSeatUnionQueryVO"> <id column="user_id" property="userId" /> <result column="account_status" property="accountStatus" /> <association property="seatInformation" javaType="com.ceiec.cad.model.SeatInformation" resultMap="com.ceiec.cad.mapper.SeatInformationMapper.BaseResultMap"> </association> </resultMap> <select id="selectOnlineUserBySeatState" resultMap="UserSeatUnionQueryVOResultMap"> SELECT u.`user_id`,u.`account_status`,s.* FROM users u, seat_information s WHERE u.`user_id` = s.`user_id` AND u.`account_status` = 1 AND u.`delete_status` = 1 AND u.`center_id` = 0 AND s.`token` IS NOT NULL AND s.`seat_state` = 1 AND s.delete_status = 1 </select> </mapper> SeatInformationMapper.xml: <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.ceiec.cad.mapper.SeatInformationMapper"> <!-- 通用查询映射结果 --> <resultMap id="BaseResultMap" type="com.ceiec.cad.model.SeatInformation"> <id column="seat_information_id" property="seatInformationId" /> //省略无关代码 </resultMap> </mapper> 二、两表关联，映射到如下类型vo（组合两表po），怎么写
上面的方案呢，假设A关联B，在vo里，相当于是把B组合进了A；我想的是，能不能新建一个vo，同时组合A和B呢，比如下面这样：
@Data public class UserSeatUnionQueryVO { private Users users; private SeatInformation seatInformation; } sql：
和方案一一样。
UsersMapper.xml：
<resultMap id="UserSeatUnionQueryVOResultMap" type="com.ceiec.cad.service.UserSeatUnionQueryVO"> <association property="users" javaType="com.ceiec.cad.model.Users" resultMap="com.ceiec.cad.mapper.UsersMapper.BaseResultMap"> </association> <association property="seatInformation" javaType="com.ceiec.cad.model.SeatInformation" resultMap="com.ceiec.cad.mapper.SeatInformationMapper.BaseResultMap"> </association> </resultMap> 注意这里的 association元素中的resultMap字段，引用了其他Mapper文件的BaseResultMap
格式是：其他Mapper文件的namespace，加上resultMap的名字。
这里附上SeatInformationMapper.xml的内容：
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.ceiec.cad.mapper.SeatInformationMapper"> <!-- 通用查询映射结果 --> <resultMap id="BaseResultMap" type="com.ceiec.cad.model.SeatInformation"> <id column="seat_information_id" property="seatInformationId" /> //省略无关 </resultMap> </mapper> 下面附上完整的usersMapper.xml的内容：
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.ceiec.cad.mapper.UsersMapper"> <!-- 通用查询映射结果 --> <resultMap id="BaseResultMap" type="com.ceiec.cad.model.Users"> <id column="user_id" property="userId" /> <result column="account_status" property="accountStatus" /> </resultMap> <resultMap id="UserSeatUnionQueryVOResultMap" type="com.ceiec.cad.service.UserSeatUnionQueryVO"> <association property="users" javaType="com.ceiec.cad.model.Users" resultMap="com.ceiec.cad.mapper.UsersMapper.BaseResultMap"> </association> <association property="seatInformation" javaType="com.ceiec.cad.model.SeatInformation" resultMap="com.ceiec.cad.mapper.SeatInformationMapper.BaseResultMap"> </association> </resultMap> <select id="selectOnlineUserBySeatState" resultMap="UserSeatUnionQueryVOResultMap"> SELECT u.*,s.* FROM users u, seat_information s WHERE u.`user_id` = s.`user_id` AND u.`account_status` = 1 AND u.`delete_status` = 1 AND u.`center_id` = 0 AND s.`token` IS NOT NULL AND s.`seat_state` = 1 AND s.delete_status = 1 </select> </mapper> 展示下效果：
可以看到，最后这种，这么写没问题。
三、官网文档
我在官网看了半天，后来才找到类似的例子：
https://mybatis.org/mybatis-3/sqlmap-xml.html
因为文档很长，上面这个图，大家可以通过打开上面的网址后，搜索 Multiple ResultSets for Association到达该位置。
