MyBatis学习（七）MyBatis关联映射之多对多映射

对于数据库中的多对多关系建议使用一个中间表来维护关系。

1.创建四张表，分别为用户表，商品表，订单表，中间表。

 1 DROP TABLE IF EXISTS `t_user`;
 2 CREATE TABLE `t_user` (
 3   `id` int(11) NOT NULL AUTO_INCREMENT,
 4   `username` varchar(18) NOT NULL,
 5   `sex` char(2) NOT NULL,
 6   `age` int(11) NOT NULL,
 7   PRIMARY KEY (`id`)
 8 ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
 9 
10 -- ----------------------------
11 -- Records of t_user
12 -- ----------------------------
13 INSERT INTO `t_user` VALUES ('1', '张三', '男', '12');
14 INSERT INTO `t_user` VALUES ('2', '王五', '男', '12');

user表

 1 -- ----------------------------
 2 -- Table structure for `t_article`
 3 -- ----------------------------
 4 DROP TABLE IF EXISTS `t_article`;
 5 CREATE TABLE `t_article` (
 6   `id` int(11) NOT NULL AUTO_INCREMENT,
 7   `name` varchar(255) NOT NULL,
 8   `price` int(11) NOT NULL,
 9   `remark` varchar(255) NOT NULL,
10   PRIMARY KEY (`id`)
11 ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
12 
13 -- ----------------------------
14 -- Records of t_article
15 -- ----------------------------
16 INSERT INTO `t_article` VALUES ('1', 'java', '100', 'java');
17 INSERT INTO `t_article` VALUES ('2', 'spring', '100', 'spring');

商品表

-- ----------------------------
-- Table structure for `t_order`
-- ----------------------------
DROP TABLE IF EXISTS `t_order`;
CREATE TABLE `t_order` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uid` int(11) NOT NULL,
  `total` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `uid` (`uid`),
  CONSTRAINT `uid` FOREIGN KEY (`uid`) REFERENCES `t_user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_order
-- ----------------------------
INSERT INTO `t_order` VALUES ('1', '1', '100');
INSERT INTO `t_order` VALUES ('2', '1', '200');

订单表

-- ----------------------------
-- Table structure for `t_item`
-- ----------------------------
DROP TABLE IF EXISTS `t_item`;
CREATE TABLE `t_item` (
  `order_id` int(11) NOT NULL,
  `article_id` int(11) NOT NULL,
  PRIMARY KEY (`order_id`,`article_id`),
  KEY `article_id` (`article_id`),
  CONSTRAINT `article_id` FOREIGN KEY (`article_id`) REFERENCES `t_article` (`id`),
  CONSTRAINT `order_id` FOREIGN KEY (`order_id`) REFERENCES `t_order` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_item
-- ----------------------------
INSERT INTO `t_item` VALUES ('1', '1');
INSERT INTO `t_item` VALUES ('2', '1');
INSERT INTO `t_item` VALUES ('1', '2');

中间表

2.创建一个项目，导入所需的jar包，导入db.perproties、log4j.properties属性文件

3.编写对应的实体类

User.java

public class User {

    private Integer id;
    private String username;
    private String sex;
    private Integer age;
    //一个用户可以有多个订单
    private List<Order> orders;
       //省略set、get、toString方法          
}

Order.java

public class Order {

    private Integer id;
    private Double total;
    //一个订单对应一个用户
    private User user;
    //一个订单可以有多个商品
    private List<Article> articles;
    //省略set、get、toString方法
}

Article.java

public class Article {

    private Integer id;
    private String name;
    private Double price;
    private String remark;
    //一种商品可以出现在多个订单中
    private List<Order> orders;
//省略get、set、toString方法
}

4.编写SQL映射文件

OrderMapper.xml

<?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.dj.mapper.OrderMapper">
    <resultMap type="com.dj.domain.Order" id="orderResultMapper">
        <id property="id" column="oid"/>
        <result property="total" column="total"/>
        <!-- 多对一关联映射 -->
        <association property="user" javaType="com.dj.domain.User">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="sex" column="sex"/>
            <result property="age" column="age"/>
        </association>
        <!-- 多对多映射 -->
        <!-- 使用查询到的column属性oid的值作为参数执行selectArticleByOrderId -->
        <collection property="articles" column="oid" fetchType="lazy"
            javaType="ArrayList" ofType="com.dj.domain.Article" 
            select="com.dj.mapper.ArticleMapper.selectArticleByOrderId"> 
            <id property="id" column="id"/>
            <result property="name" column="name"/>
            <result property="price" column="price"/>
            <result property="remark" column="remark"/>
        </collection>
    </resultMap>
    <!-- 查询结果有两个id列，所以把order的id取一个别名oid -->
    <select id="selectOrderById" parameterType="int" resultMap="orderResultMapper">
        select u.*,o.id as oid ,o.total     
        from t_user u,t_order o
        where u.id=o.uid and o.id=#{id}
    </select>
</mapper>

ArticleMapper.xml

<?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.dj.mapper.ArticleMapper">
    <select id="selectArticleByOrderId" parameterType="int"
        resultType="com.dj.domain.Article">
            select * from t_article where id in(
                select article_id from t_item where order_id=#{id}
            )
            </select>
</mapper>

5.mapper接口对象

public interface OrderMapper {
Order selectOrderById(int id);
}

6.测试类

public class SelectOrderTest {
    public static void main(String[] args) throws Exception {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = factory.openSession();
        //获得接口的代理对象
        OrderMapper om = session.getMapper(OrderMapper.class);
        Order order = om.selectOrderById(1);
        System.out.println(order);
        User user = order.getUser();
        System.out.println(user);
        List<Article> articles = order.getArticles();
        for (Article article : articles) {
            System.out.println(article);    
        }  
    }
}

　　流程：

　　得到一个OrderMapper接口的代理对象，调用里面的selectOrderById(int id)方法，找到OrderMapper.xml里面 id为selectOrderById的select属性，返回的是一个resultMap，找到这个resultMap，这个resultMap返回的是一个Order类型。

　　执行id为selecOrderById的select属性里面的SQL语句结果如下图