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
('', '', 'title1', 'content1'),
('', '', 'title2', 'content2'),
('', '', 'title3', 'content3'),
('', '', '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" />

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