关联有嵌套查询和嵌套结果两种方式，本文是按照 嵌套结果 这种方式来说明的
上一章介绍了多对一的关系，用到了，这是一个复杂类型的关联。我们选择一个示例来回顾下,比如：一个博客有一个用户，关联映射就工作于这种结果之上。首先看下，我们在本文中要用到的表结构字段:
博客
blog : id title author_id
作者
author: id username password email bio favourite_section
文章
post :id blog_id author_id created_on section subject draft body
评论
comment : id post_id name comment
标签
T : id name
我们把一个博客和一个用户关联在一起，就像：
<select id="selectBlog" parameterType="int" resultMap="blogResult"> select b.id as blog_id, b.title as blog_title, b.author_id as blog_author_id a.id as author_id, a.username as author_username, a.password as author_passowrd, a.email as auhtor_email, a.bio as author_bio from blog b left outer join author a on b.author_id=a.id where b.id= </select>
注意这个联合查询，以及所有结果被唯一而清晰的名字而重命名。这使得映射非常简单。现在我们可以映射这个结果：
<resultMap id="blogResult" type="Blog"> <id property="id" column="blog_id"/> <result property="title" column="blog_title"/> <association property="author" column="blog_author_id" javaType="Author" resultMap="authorResult"/></resultMap><resultMap id="authorResult" type="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> <result property="email" column="author_email"/> <result property="bio" column="author_bio"/></resultMap>
在上面的例子中，你可以看到博客的作者关联authorResult 结果映射来加载作者实例。 上面的实例中，用了外部的结果映射元素来映射关联。这使得Author结果映射可以重用。然而，你不需要重用它的话，或者你仅仅引用你所有的结果映射到一个单独描述的结果映射中。你可以嵌套结果映射。这里给出使用这种方式的相同示例：
<resultMap id="blogResult" type="Blog"> <id property="id" column="blog_id"/> <result property="title" column="blog_title"/> <association property="author" column="blog_author_id" javaType="Author" > <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> <result property="email" column="author_email"/> <result property="bio" column="author_bio"/> </association></resultMap>
上面你已经看到了如何处理有一个类型的关联.但是“有很多个”是怎样的呢？，也就是集合类型， 本文的主要工作是来说这个的 。
集合
相对于关联来说，集合映射多了一个属性”ofType“.这个属性用来区分 JavaBean（或字段）属性类型 和 集合包含的类型 来说是很重要的. ，ofType用来表示集合包含类型。
<collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <result property="body" column="post_body"/> </collection>
集合元素的作用和关联几乎是相同的。我们来继续上面的示例，一个博客只有一个作者。但是博客有很多文章，在博客类中，这可以由下面的写法来表示：
private List posts;
这一次联合博客表和文章表（一个blog_id可以对应很多的文章）SQL如下：
<select id="selectBlog" parameterType="int" resultMap="blogResult"> select b.id as blog_id , b.title as blog_title, b.author_id as blog_author_id, p.id as post_id, p.subject as post_subject, p.body as post_body from blog b left outer join post p on b.id=p.blog_id where b.id=</select>
现在用“文章映射集合”来映射 “博客“，可以简单写为：
<resultMap id="blogResult" type="Blog"> <id property="id" column="blog_id"/> <result property="title" column="blog_title"/> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <result property="body" column="post_body"/> </collection></resultMap>
高级关联和集合映射还有很多要琢磨的地方。就让面介绍的集合映射，稍微的整理下（一个博客下面，有很多文章）.
首先创建下，需要用到的表格，以及向其中插入一些数据.
create table author(id int (11) not null auto_increment, username varchar(20) not null, password varchar(20) not null, email varchar(20) not null, bio varchar(20) not null, favourite_section varchar(20) not null, primary key(id) )ENGINE=InnoDB DEFAULT CHARSET=utf8;insert into author(id,username,password,email,bio,favourite_section) values(1001,'林枫','123456','824156593@qq.com','合肥的小男孩','旅游');create table blog (id int (11) not null auto_increment, title varchar(20) not null, author_id int(11) not null, primary key(id)) ENGINE=InnoDB DEFAULT CHARSET=utf8; insert into blog(id,title,author_id) values(1,'小说博客',1001); create table post(id int (11) not null auto_increment, blog_id int(11) not null, author_id int(11) not null, created_on date not null, section varchar(20) not null, subject varchar(20) not null, draft varchar(20) not null, body varchar(20) not null, primary key(id) )ENGINE=InnoDB DEFAULT CHARSET=utf8; insert into post(id,blog_id,author_id,created_on,section,subject,draft,body)values(1,1,1001,now(),'旅游','玄幻','草稿','绝世武神'); insert into post(id,blog_id,author_id,created_on,section,subject,draft,body)values(2,1,1001,now(),'旅游','玄幻','草稿','大主宰');insert into post(id,blog_id,author_id,created_on,section,subject,draft,body)values(3,1,1001,now(),'旅游','玄幻','草稿','灵域');
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在贴JAVA代码之前，先看下目录结构吧：
作者类 Author.javapackage com.mybatis.model;/** * 作者类 * @author Administrator * */public class Author { private int id; private String username; private String password; private String email; private String bio; private String favourite_section; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getUsername() { return username; } public void setUsername(String username) { this.username = username; } public String getPassword() { return password; } public void setPassword(String password) { this.password = password; } public String getEmail() { return email; } public void setEmail(String email) { this.email = email; } public String getBio() { return bio; } public void setBio(String bio) { this.bio = bio; } public String getFavourite_section() { return favourite_section; } public void setFavourite_section(String favouriteSection) { favourite_section = favouriteSection; }}Blog.javapackage com.mybatis.model;import java.util.List;/** * 博客类 * @author Administrator * */public class Blog { private int id; private String title; private Author author; private List<Post> posts; public List<Post> getPosts() { return posts; } public void setPosts(List<Post> posts) { this.posts = posts; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getTitle() { return title; } public void setTitle(String title) { this.title = title; } public Author getAuthor() { return author; } public void setAuthor(Author author) { this.author = author; }}文章类Post.,javapackage com.mybatis.model;import java.util.Date;/** * 文章类 * @author Administrator * */public class Post { private int id; private int blog_id; private int author_id; private Date created_on; private String section; private String subject; private String draft; private String body; public int getId() { return id; } public void setId(int id) { this.id = id; } public int getBlog_id() { return blog_id; } public void setBlog_id(int blogId) { blog_id = blogId; } public int getAuthor_id() { return author_id; } public void setAuthor_id(int authorId) { author_id = authorId; } public Date getCreated_on() { return created_on; } public void setCreated_on(Date createdOn) { created_on = createdOn; } public String getSection() { return section; } public void setSection(String section) { this.section = section; } public String getSubject() { return subject; } public void setSubject(String subject) { this.subject = subject; } public String getDraft() { return draft; } public void setDraft(String draft) { this.draft = draft; } public String getBody() { return body; } public void setBody(String body) { this.body = body; }}
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总配置文件
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration> <typeAliases> <typeAlias type="com.mybatis.model.Blog" alias="Blog"/> <typeAlias type="com.mybatis.model.Post" alias="Post"/> </typeAliases> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://127.0.0.1:3306/test"/> <property name="username" value="root"/> <property name="password" value="123456"/> </dataSource> </environment> </environments> <mappers> <mapper resource="com/mybatis/model/Blog.xml"/> </mappers></configuration>Blog.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.mybatis.dao.IBlogDao"><resultMap id="blogResult" type="Blog"> <id property="id" column="blog_id"/> <result property="title" column="blog_title"/> <collection property="posts" ofType="Post"> <id property="id" column="post_id"/> <result property="subject" column="post_subject"/> <result property="body" column="post_body"/> </collection></resultMap><select id="selectBlog" parameterType="int" resultMap="blogResult"> select b.id as blog_id , b.title as blog_title, b.author_id as blog_author_id, p.id as post_id, p.subject as post_subject, p.body as post_body from blog b left outer join post p on b.id=p.blog_id where b.id=#{id} </select></mapper>测试类Test.javapackage com.mybatis.test;import java.io.IOException;import java.util.List;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import com.mybatis.dao.IBlogDao;import com.mybatis.model.Blog;import com.mybatis.model.Post;public class Test { /*** * 获得MyBatis SqlSessionFactory * SqlSessionFactory 负责创建SqlSession ,一旦创建成功，就可以用SqlSession实例来执行映射语句 * ，commit,rollback,close等方法 * @return */ private static SqlSessionFactory getSessionFactory(){ SqlSessionFactory sessionFactory=null; String resource="configuration.xml"; try { sessionFactory=new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource)); } catch (IOException e) { e.printStackTrace(); } return sessionFactory; } /** * main 方法 * @param args */ public static void main(String[] args) { SqlSession session=getSessionFactory().openSession(); try { IBlogDao blogDao=session.getMapper(IBlogDao.class); Blog blog=blogDao.selectBlog(1); List<Post> postList=blog.getPosts(); for(Post post:postList){ System.out.println(post.getBody()); } } catch (Exception e) { e.printStackTrace(); } finally{ session.close(); } }}运行后结果如下：DEBUG [com.mybatis.dao.IBlogDao.selectBlog] - ooo Using Connection [com.mysql.jdbc.Connection@e00ed0]DEBUG [com.mybatis.dao.IBlogDao.selectBlog] - ==> Preparing: select b.id as blog_id , b.title as blog_title, b.author_id as blog_author_id, p.id as post_id, p.subject as post_subject, p.body as post_body from blog b left outer join post p on b.id=p.blog_id where b.id=? DEBUG [com.mybatis.dao.IBlogDao.selectBlog] - ==> Parameters: 1(Integer)绝世武神大主宰灵域
转载网址：http://blog.csdn.net/Smile_Miracle/article/details/53287671
