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mybatis ---- 级联查询一对多（集合映射）

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

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原文地址：https://www.cnblogs.com/wuyida/p/6300374.html

mybatis ---- 级联查询 一对多 （集合映射）

集合

mybatis ---- 级联查询一对多（集合映射）