myBatis系列之四：关联数据的查询

myBatis系列之三：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.

<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) {       log.info("{} - {}, 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" />

1. <resultMap type="com.john.hbatis.model.Article" id="articleList">  
2.     <id column="a_id" property="id" />  
3.     <result column="title" property="title" />  
4.     <result column="content" property="content" />  
5.       
6.     <association property="user" javaType="User"><!-- user属性映射到User类 -->  
7.         <id column="id" property="id" />  
8.         <result column="name" property="name" />  
9.         <result column="address" property="address" />  
10.     </association>  
11. </resultMap>  
12.   
13. <select id="getArticlesByUserId" parameterType="int" resultMap="articleList">  
14.     select u.id, u.name, u.age, u.address, a.id a_id, a.title, a.content  
15.     from article a  
16.     inner join user u  
17.     on a.user_id=u.id and u.id=#{id}  
18. </select>

原标题：myBatis系列之四：关联数据的查询

关键词：mybatis