用户表和订单表主键名一样，用as重命名解决映射问题

说明：user表和orders表(一个用户可以有多个订单；一个订单只对应一个用户)

<resultMap type="Orders" id="order">
  <id column="id" property="id" />
  <result column="user_id" property="userId" />
  <!-- <result column="id" property="id"/> -->
  <association property="user" javaType="User">
   <id column="uid" property="id" />
   <result column="username" property="username" />
  </association>
 </resultMap>
 <select id="selectOrders" resultMap="order">
  select
  o.id,
  o.user_id,
  o.number,
  o.createtime,
  u.username,
  u.id as uid
  from orders o
  left join
  user u
  on o.user_id=u.id
 </select>