mybatis 一对多嵌套查询的实现

public class Order {
    private int id;
    private String productName;
    // 省略其他属性...

    // 定义User的外键引用
    private int userId;
    private User user; // 可选，取决于是否需要双向关联

    // 省略getter和setter方法...
}

public class User {
    private int id;
    private String username;
    // 省略其他属性...

    // 定义一个Order集合属性
    private List<Order> orders;

    // 省略getter和setter方法...
}

<mapper namespace="com.example.mapper.UserMapper">
    <!-- 查询用户基本信息 -->
    <select id="getUserById" parameterType="int" resultType="com.example.entity.User">
        SELECT * FROM user WHERE id = #{id}
    </select>

    <!-- 一对多关联查询用户的所有订单 -->
    <select id="getUserAndOrdersById" parameterType="int" resultType="com.example.entity.User">
        SELECT u.*, o.* FROM user u
        LEFT JOIN order o ON u.id = o.user_id
        WHERE u.id = #{id}
    </select>

    <!-- 使用association标签配置一对一关系 -->
    <resultMap id="userResultMap" type="com.example.entity.User">
        <id property="id" column="id"/>
        <result property="username" column="username"/>
        <!-- 省略其他字段映射 -->

        <!-- 一对多关联映射 -->
        <collection property="orders" ofType="com.example.entity.Order">
            <id property="id" column="order_id"/>
            <result property="productName" column="product_name"/>
            <!-- 省略其他字段映射 -->
            <!-- 设置关联外键 -->
            <association property="user" javaType="com.example.entity.User" column="user_id" select="com.example.mapper.UserMapper.getUserById"/>
        </collection>
    </resultMap>

    <!-- 使用resultMap进行查询 -->
    <select id="getUserWithResultMapById" parameterType="int" resultMap="userResultMap">
        SELECT * FROM user u
        LEFT JOIN order o ON u.id = o.user_id
        WHERE u.id = #{id}
    </select>
</mapper>

import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import java.util.List;

@TableName("user")
public class User {
    @TableId("id")
    private Long id;
    private String username;

    // 通过@TableField注解关联Order表
    @TableField(exist = false)
    private List<Order> orders;

    // getter和setter方法...
}

import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableName;

@TableName("order")
public class Order {
    @TableId("id")
    private Long id;
    private String productName;
    @TableField("user_id")
    private Long userId;

    // getter和setter方法...
}

import com.baomidou.mybatisplus.core.conditions.Wrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.extension.service.IService;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;

@Service
public class UserServiceImpl extends ServiceImpl<UserMapper, User> implements IService<User> {

    public User getUserAndOrders(Long userId) {
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();
        queryWrapper.eq("id", userId);
        // 使用lambda表达式加载关联的orders
        queryWrapper.lambda().with(User::getOrders);
        return baseMapper.selectOne(queryWrapper);
    }
}