Mybatis源码分析(十三) - 关联查询之多对多

我的理解是，多对多其实就是两个一对多。

嵌套结果：

示例代码：

<select id="selectUserRole" resultMap="userRoleInfo">
    select a.id, 
          a.user_name,
          a.real_name,
          a.sex,
          a.mobile,
          a.note,
          b.role_id,
          c.role_name,
          c.note role_note
    from t_user a,
         t_user_role b,
         t_role c
    where a.id = b.user_id AND 
          b.role_id = c.id
     </select>

<resultMap type="TUser" id="userRoleInfo" extends="BaseResultMap">
    <collection property="roles" ofType="TRole" columnPrefix="role_">
      <result column="id" property="id" />
      <result column="Name" property="roleName" />
      <result column="note" property="note" />
    </collection>
</resultMap>

嵌套查询：

<select id="selectUserbyrole" resultMap="userRoleInfo1">
  select * from t_role a
  </select>

<resultMap id="userRoleInfo1" extends="BaseResultMap" type="TRole">
    <collection property="users" column="id"
      select= "com.enjoylearning.mybatis.mapper.TUserMapper.selectUsers12" fetchType="lazy"></collection>
  </resultMap>

<select id="selectUsers12" resultMap="BaseResultMap">
    SELECT a.id,  a.user_name,
          a.real_name,
          a.sex,
          a.mobile,
          a.note
    FROM t_user a,t_user_role b
    WHERE a.id = b.user_id AND b.role_id=#{id}
</select>

@Test
  // 多对多
  public void testManyToMany1() {
    // 2.获取sqlSession
    SqlSession sqlSession = sqlSessionFactory.openSession();
    // 3.获取对应mapper
    TRoleMapper mapper = sqlSession.getMapper(TRoleMapper.class);
    // 4.执行查询语句并返回结果
    // ----------------------
    List<TRole> list = mapper.selectUserbyrole();
    for (TRole tUser : list) {
      System.out.println(tUser.getUsers().size());
    }
  }