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