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<association></association>，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=#{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"/>
<!-- 和一个用户关联,Blog 类里面属性时author,关联的列是原先的blog.author_id-->
<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"/>
<!-- 和一个用户关联,Blog 类里面属性时author,关联的列是原先的blog.author_id-->
<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<Post> 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=#{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');
在贴JAVA代码之前，先看下目录结构吧：
作者类 Author.java
package 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; //个人资料
private String favourite_section; //最喜欢的。。
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.java
package 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; //博客类有很多文章， 与post表中的blog_id对应
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.,java
package 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;
}
}
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>
<!-- 给实体类去一个别名 -->
<typeAlias type="com.mybatis.model.Blog" alias="Blog"/>
<typeAlias type="com.mybatis.model.Post" alias="Post"/>
</typeAliases>
<!-- 数据源配置，这里用MySQL数据库 -->
<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">
<!-- 用文章映射集合来映射博客 -->
<resultMap id="blogResult" type="Blog">
<id property="id" column="blog_id"/>
<result property="title" column="blog_title"/>
<!--文章集合 -->
<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.java
package 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
