mybatis ---- 級聯查詢 一對多 （集合映射）

原文鏈接

關聯有嵌套查詢和嵌套結果兩種方式，本文是按照嵌套結果這種方式來說明的

上一章介紹了多對一的關係，用到了<association></association>，這是一個複雜類型的關聯。我們選擇一個示例來回顧下,比如：一個博客有一個用戶，關聯映射就工作於這種結果之上。首先看下，我們在本文中要用到的表結構字段:

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

評論
comment : id   post_id  name  comment

標籤
T : id name

我們把一個博客和一個用戶關聯在一起，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>

注意這個聯合查詢，以及所有結果被唯一而清晰的名字而重命名。這使得映射非常簡單。現在我們可以映射這個結果：

<resultMap id="blogResult" type="Blog">
     <id property="id" column="blog_id"/>
      <result property="title" column="blog_title"/>

      <!-- 和一個用戶關聯,Blog 類裏面屬性時author,關聯的列是原先的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關聯authorResult 結果映射來加載作者實例。  上面的實例中，用了外部的結果映射元素來映射關聯。這使得Author結果映射可以重用。然而，你不需要重用它的話，或者你僅僅引用你所有的結果映射到一個單獨描述的結果映射中。你可以嵌套結果映射。這裏給出使用這種方式的相同示例：

<resultMap id="blogResult" type="Blog">
     <id property="id" column="blog_id"/>
      <result property="title" column="blog_title"/>

      <!-- 和一個用戶關聯,Blog 類裏面屬性時author,關聯的列是原先的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>

上面你已經看到了如何處理有一個類型的關聯.但是“有很多個”是怎樣的呢？，也就是集合類型，本文的主要工作是來說這個的。

集合

相對於關聯來說，集合映射多了一個屬性”ofType“.這個屬性用來區分JavaBean（或字段）屬性類型和集合包含的類型來說是很重要的. ，ofType用來表示集合包含類型。

<collection property="posts"  ofType="Post">
       <id property="id" column="post_id"/>
       <result property="subject" column="post_subject"/>
       <result property="body" column="post_body"/>
 </collection>

集合元素的作用和關聯幾乎是相同的。我們來繼續上面的示例，一個博客只有一個作者。但是博客有很多文章，在博客類中，這可以由下面的寫法來表示：

private List<Post> posts;

這一次聯合博客表和文章表（一個blog_id可以對應很多的文章）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>

現在用“文章映射集合”來映射 “博客“，可以簡單寫爲：

<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>

高級關聯和集合映射還有很多要琢磨的地方。就讓面介紹的集合映射，稍微的整理下（一個博客下面，有很多文章）.

首先創建下，需要用到的表格，以及向其中插入一些數據. 

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,'林楓','123456','[email protected]','合肥的小男孩','旅遊');



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,'小說博客',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(),'旅遊','玄幻','草稿','絕世武神');

insert into post(id,blog_id,author_id,created_on,section,subject,draft,body)
values(2,1,1001,now(),'旅遊','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(),'旅遊','玄幻','草稿','靈域');

在貼JAVA代碼之前，先看下目錄結構吧：

作者類 Author.java

package com.mybatis.model;
/**
 * 作者類
 * @author Administrator
 *
 */
public class Author {

    private int id;
    private String username;
    private String password;
    private String email;
    private String bio; //個人資料
    private String favourite_section; //最喜歡的。。

    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;

/**
 * 博客類
 * @author Administrator
 *
 */
public class Blog {
    private int id;
    private String title;
    private Author author;
    private List<Post> posts; //博客類有很多文章， 與post表中的blog_id對應

    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;
    }

}

文章類Post.,java

package com.mybatis.model;

import java.util.Date;

/**
 * 文章類
 * @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;
    }

}

總配置文件

<?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>
    <!-- 給實體類去一個別名  -->
        <typeAlias type="com.mybatis.model.Blog" alias="Blog"/>
        <typeAlias type="com.mybatis.model.Post" alias="Post"/>
  </typeAliases>

  <!-- 數據源配置，這裏用MySQL數據庫 -->
  <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">

<!-- 用文章映射集合來映射博客 -->
<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>

測試類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 {

    /***
     * 獲得MyBatis SqlSessionFactory
     * SqlSessionFactory 負責創建SqlSession ,一旦創建成功，就可以用SqlSession實例來執行映射語句
     * ，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();
        }
    }
}

運行後結果如下：

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)
絕世武神
大主宰
靈域