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.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) {  
            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" />

<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>