// 用戶實體（一的一方）
@Data
public class User {
    private Integer id;
    private String username;
    // 一對多：存放當前用戶的所有訂單
    private List<Order> orderList;
}
// 訂單實體（多的一方）
@Data
public class Order {
    private Integer id;
    private String orderNo;
    // 多對一：存放訂單所屬的用戶
    private User user;
}

<!-- 單表查詢：resultType 直接映射，無需配置resultMap -->
<select id="selectById" resultType="com.itheima.pojo.User">
    select * from tb_user where id = #{id}
</select>

<?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.itheima.mapper.OrderMapper">
    <!-- 一對一結(jié)果映射 -->
    <resultMap id="orderResultMap" type="order" autoMapping="true">
        <id property="id" column="id"/>
        <result property="orderNo" column="order_no"/>
        <!-- 一對一關(guān)聯(lián)配置：延遲加載用戶信息 -->
        <association
            property="user"
            javaType="user"
            select="com.itheima.mapper.UserMapper.selectById"
            column="user_id"
            fetchType="lazy">
        </association>
    </resultMap>
    <!-- 根據(jù)id查詢訂單 -->
    <select id="selectById" resultMap="orderResultMap">
        select * from tb_order where id = #{id}
    </select>
</mapper>

<?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.itheima.mapper.UserMapper">
    <!-- 1. 定義復(fù)雜映射規(guī)則：id唯一標識，type映射實體類，autoMapping自動匹配字段 -->
    <resultMap id="userResultMap" type="user" autoMapping="true">
        <!-- 2. 主鍵映射（規(guī)范寫法，可省略） -->
        <id property="id" column="id"/>
        <!-- 普通字段映射（autoMapping=true可自動匹配，可省略） -->
        <result property="username" column="username"/>
        <!-- 3. 核心：一對多關(guān)聯(lián)映射，延遲加載配置 -->
        <collection
            property="orderList"
            ofType="order"
            select="com.itheima.mapper.OrderMapper.findByUid"
            column="id"
            fetchType="lazy">
        </collection>
    </resultMap>
    <!-- 4. 查詢用戶，綁定上面的映射規(guī)則 -->
    <select id="selectById" resultMap="userResultMap">
        select * from tb_user where id = #{id}
    </select>
</mapper>

<settings>
    <!-- 開啟延遲加載總開關(guān) -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 關(guān)閉積極加載（3.4.1后默認false，配置更保險） -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>

<mapper namespace="com.itheima.mapper.OrderMapper">
    <!-- 根據(jù)用戶id查詢訂單：供一對多延遲加載調(diào)用 -->
    <select id="findByUid" resultType="com.itheima.pojo.Order">
        select * from tb_order where user_id = #{userId}
    </select>
</mapper>

// UserMapper
public interface UserMapper {
    User selectById(Integer id);
}
// OrderMapper
public interface OrderMapper {
    List<Order> findByUid(Integer userId);
    Order selectById(Integer id);
}

@Test
public void testOneToOne(){
    // 1. 僅查詢訂單：只執(zhí)行1條SQL → select * from tb_order where id=1
    Order order = orderMapper.selectById(1);
    System.out.println("訂單編號：" + order.getOrderNo());
    // 2. 調(diào)用user：觸發(fā)一對一延遲加載，查詢用戶信息
    System.out.println("訂單所屬用戶：" + order.getUser());
}

@Test
public void testLazyLoad(){
    // 1. 僅查詢用戶：只執(zhí)行1條SQL → select * from tb_user where id=1
    User user = userMapper.selectById(1);
    System.out.println("查詢到用戶：" + user.getUsername());
    // 2. 未使用訂單數(shù)據(jù)：不執(zhí)行訂單查詢SQL
    // 3. 調(diào)用orderList：觸發(fā)延遲加載，執(zhí)行訂單SQL → select * from tb_order where user_id=1
    System.out.println("用戶訂單：" + user.getOrderList());
}