认识达内从这里开始

增删改查是基于单表的查询，如果联表查询，返回的是复合对象，需要用association关键字来处理。

如User发表Article，每个用户可以发表多个Article，他们之间是一对多的关系。

1.创建Article表，并插入测试数据：

-- Drop the table if exists 

DROP TABLE IF EXISTS `Article`; 

-- Create a table named 'Article' 

CREATE TABLE `Article` ( 

`id` int NOT NULL AUTO_INCREMENT, 

`user_id` int NOT NULL, 

`title` varchar(100) NOT NULL, 

`content` text NOT NULL, 

PRIMARY KEY (`id`) 

) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8; 

-- Add several test records 

INSERT INTO `article` 

VALUES 

('1', '1', 'title1', 'content1'), 

('2', '1', 'title2', 'content2'), 

('3', '1', 'title3', 'content3'), 

('4', '1', 'title4', 'content4');

2. com.john.hbatis.model.Article类：

public class Article { 

private int id; 

private User user; 

private String title; 

private String content; 

// Getters and setters are omitted 

} 

3.在IUserMapper中添加：

List<Article> getArticlesByUserId(int id);

4.在User.xml中添加：

<resultMap type="com.john.hbatis.model.Article" id="articleList"> 

<id column="a_id" property="id" /> 

<result column="title" property="title" /> 

<result column="content" property="content" /> 

<association property="user" javaType="User"><!-- user属性映射到User类--> 

<id column="id" property="id" /> 

<result column="name" property="name" /> 

<result column="address" property="address" /> 

</association> 

</resultMap> 

<select id="getArticlesByUserId" parameterType="int" resultMap="articleList"> 

select u.id, u.name, u.age, u.address, a.id a_id, a.title, a.content 

from article a 

inner join user u 

on a.user_id=u.id and u.id=#{id} 

</select>

5.测试方法：

@Test 

public void getArticlesByUserIdTest() { 

SqlSession session = sqlSessionFactory.openSession(); 

try { 

IUserMapper mapper = session.getMapper(IUserMapper.class); 

List<Article> articles = mapper.getArticlesByUserId(1); 

for (Article article : articles) { 

#("{} - {}, author: {}", article.getTitle(), article.getContent(), article.getUser().getName()); 

} 

} finally { 

session.close(); 

} 

} 

附：除了在association标签内定义字段和属性的映射外，还可以重用User的resultMap:

<association property="user" javaType="User" resultMap="userList" />

<resultMap type="com.john.hbatis.model.Article" id="articleList"> 

<id column="a_id" property="id" /> 

<result column="title" property="title" /> 

<result column="content" property="content" /> 

<association property="user" javaType="User"><!-- user属性映射到User类--> 

<id column="id" property="id" /> 

<result column="name" property="name" /> 

<result column="address" property="address" /> 

</association> 

</resultMap> 

<select id="getArticlesByUserId" parameterType="int" resultMap="articleList"> 

select u.id, u.name, u.age, u.address, a.id a_id, a.title, a.content 

from article a 

inner join user u 

on a.user_id=u.id and u.id=#{id} 

</select>

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