Mybatis一对多查询列表属性处理示例详解

<resultMap id="userResultMap" type="User">
  <id column="user_id" property="id"/>
  <result column="user_name" property="name"/>
  <collection property="roles" ofType="Role">
    <id column="role_id" property="id"/>
    <result column="role_name" property="name"/>
  </collection>
</resultMap>

<resultMap id="userResultMap" type="User">
  <id column="user_id" property="id"/>
  <result column="user_name" property="name"/>
  <collection property="roles" ofType="Role" columnPrefix="role_">
    <id column="role_id" property="id"/>
    <result property="name"/>
  </collection>
</resultMap>

public class User {
    private Integer id;
    private String name;
    private Integer age;
    private List<Order> orders;

    // 省略 getter 和 setter 方法
}

public class Order {
    private Integer id;
    private String orderNo;
    private Date createTime;

    // 省略 getter 和 setter 方法
}

<select id="getUserOrdersById" resultMap="userResultMap">
  SELECT u.id, u.name, u.age, o.id AS order_id, o.order_no, o.create_time
  FROM user u
  INNER JOIN `order` o ON u.id = o.user_id
  WHERE u.id = #{id}
</select>
<resultMap id="userResultMap" type="com.example.User">
  <id property="id" column="id" />
  <result property="name" column="name" />
  <result property="age" column="age" />
  <collection property="orders" ofType="com.example.Order" resultMap="orderResultMap" />
</resultMap>
<resultMap id="orderResultMap" type="com.example.Order">
  <id property="id" column="order_id" />
  <result property="orderNo" column="order_no" />
  <result property="createTime" column="create_time" />
</resultMap>

<select id="getUserOrdersById" resultMap="userResultMap">
  SELECT u.id, u.name, u.age
  FROM user u
  WHERE u.id = #{id}
</select>
<select id="getOrderByUserId" resultMap="orderResultMap">
  SELECT o.id, o.order_no, o.create_time
  FROM `order` o
  WHERE o.user_id = #{userId}
</select>
<resultMap id="orderResultMap" type="com.example.Order">
  <id property="id" column="order_id" />
  <result property="orderNo" column="order_no" />
  <result property="createTime" column="create_time" />
</resultMap>
<resultMap id="userResultMap" type="com.example.User">
  <id property="id" column="id" />
  <result property="name" column="name" />
  <result property="age" column="age" />
  <collection property="orders" ofType="com.example.Order" resultMap="orderResultMap" select="getOrderByUserId" column="id">
      <!-- 这里使用 column="id"，指定了内层 select 语句的参数值为外层查询语句结果集中的 id 属性值(即用户ID) -->
  </collection>
</resultMap>

public class OrderQuery {
    private Long userId; // 用户id
    private Integer status; // 订单状态
    // getter和setter方法
}

<select id="getOrdersByUserId" resultType="Order">
    SELECT *
    FROM `order`
    WHERE user_id = #{userId}
    AND status = #{status}
</select>
<resultMap id="userMap" type="User">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <!-- 注意这里的association标签的select属性使用了OrderQuery对象的属性 -->
    <result property="orders" column="id" 
            select="com.example.mapper.OrderMapper.getOrdersByUserId">
        <association property="param" javaType="OrderQuery">
            <result property="userId" column="id"/>
            <result property="status" value="1"/> <!-- 这里传递的是固定值，也可以替换为动态的表达式 -->
        </association>
    </result>
</resultMap>

package com.example;

public class Order {
  private int orderId;
  private String orderName;
  private Customer customer;

  // getters and setters for orderId, orderName, customer
}

package com.example;

public class Customer {
  private int customerId;
  private String customerName;

  // getters and setters for customerId, customerName
}

<resultMap id="orderMap" type="com.example.Order">
  <id property="orderId" column="order_id"/>
  <result property="orderName" column="order_name"/>
  <association property="customer" javaType="com.example.Customer">
    <id property="customerId" column="customer_id"/>
    <result property="customerName" column="customer_name"/>
  </association>
</resultMap>