MyBatis 集合属性

Question

我有以下表格：

CREATE TABLE users (
  id VARCHAR (255) NOT NULL,
  name VARCHAR (255) NOT NULL,
  create_date TIMESTAMP NOT NULL,
  email VARCHAR (255) NOT NULL,
  password VARCHAR (255) NOT NULL,
  last_visit TIMESTAMP,
  avatar_id VARCHAR (255)
);

CREATE TABLE user_friends (
  user_id VARCHAR (255) NOT NULL,
  friend_id VARCHAR (255) NOT NULL
);

CREATE TABLE file (
  id VARCHAR (255) NOT NULL,
  file_path VARCHAR (255) NOT NULL,
  create_date TIMESTAMP NOT NULL,
  user_id VARCHAR (255) NOT NULL
);

我想 select 用户和他所有的朋友，所以我制作了 mapper

<sql id="coreUserAttributes">
      users.id,
      users.name,
      users.last_visit as lastVisit
</sql>

<select id="getUser" resultMap="UserRM">
    SELECT
    <include refid="coreUserAttributes"/>,
    f.id as friendId,
    f.name as friendName
    FROM users
    LEFT JOIN user_friends ON users.id = user_friends.user_id
    LEFT JOIN users f ON f.id = user_friends.friend_id
    <where>
        users.id = #{id}
    </where>
</select>

我已经尝试运行 select - 它没有任何问题。问题出在 ResultMap

如果我插入没有 <collection> 标签的 resultMap，一切正常和映射器 returns 我的用户对象

<resultMap id="UserRM" type="User">
    <id column="id"/>
</resultMap>

但是当我插入 <collection> 标签时

<resultMap id="UserRM" type="User">
    <id column="id"/>
    <collection property="friends" ofType="User" javaType="ArrayList">
        <id column="friendId"/>
        <result column="friendName" property="name"/>
    </collection>
</resultMap>

映射器调用的结果为空。我在控制台中没有看到任何错误。如何加载用户及其所有朋友？

P.S 另外，有趣的是，我在日志中看到以下文本

2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - ==>  Preparing: SELECT users.id, users.name, users.last_visit as lastVisit , f.id as friendId, f.name as friendName FROM users LEFT JOIN user_friends ON users.id = user_friends.user_id LEFT JOIN users f ON f.id = user_friends.friend_id WHERE users.id = ? 
2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - ==>  Preparing: SELECT users.id, users.name, users.last_visit as lastVisit , f.id as friendId, f.name as friendName FROM users LEFT JOIN user_friends ON users.id = user_friends.user_id LEFT JOIN users f ON f.id = user_friends.friend_id WHERE users.id = ? 
2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - ==> Parameters: 1(String)
2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - ==> Parameters: 1(String)
2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - <==      Total: 1
2017-01-03 22:29:19 DEBUG u.s.network.query.UserMapper.getUser - <==      Total: 1

据我了解，select正常，但是在mybatis构建对象时，出现了一些问题。

Answer 1

解决方案非常简单：我已将别名添加到此列，一切都开始正常工作

<sql id="coreUserAttributes">
      users.id AS id,
      users.name AS name,
      users.last_visit as lastVisit
</sql>

此外，如果没有至少一个属性映射到您的 class - 结果将为空值

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