ibatis多对一表关联映射的解决过程

1、问题：当查询新闻的时候我们一般情况下需要新闻的类型以及作者的信息。根据表设计的规范我们要把文章，文章类型，作者信息分别设计成一张表，其中文章中分别存在指向文章类型和作者信息的外键。我们的问题就是怎么使用ibatis把表空间数据映射为java的实体类属性。

2、表DDL，
   用户表
   CREATE TABLE `users` (
`id` int(11) NOT NULL auto_increment,
`username` varchar(25) default NULL,
’password` varchar(32) default NULL,
`gender` int(11) default '1',
`birth` date default NULL,
  `user_type` int(11) default NULL,
PRIMARY KEY  (`id`)
  ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;
 
  文章类型表
  CREATE TABLE `article_class` (
  `acid` int(11) NOT NULL auto_increment,
  `name` varchar(25) default NULL,
  `class_desc` varchar(255) default NULL,
  PRIMARY KEY  (`acid`)
  ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

  文章表
  CREATE TABLE `article` (
  `aid` int(11) NOT NULL auto_increment,
  `title` varchar(36) default NULL,
  `content` text,
  `classid` int(11) default NULL,
  `userid` int(11) default NULL,
  PRIMARY KEY  (`aid`),
  KEY `classid` (`classid`),
  KEY `userid` (`userid`),
  CONSTRAINT `article_ibfk_1` FOREIGN KEY (`classid`) REFERENCES `article_class` (`acid`),
  CONSTRAINT `article_ibfk_2` FOREIGN KEY (`userid`) REFERENCES `users` (`id`)
  ) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;

3、实体类

   import java.sql.Date;

	public class User {
		private Integer id;
		private String username;
		private String password;
		private int gender;
		private Date birth;
		private int userType = 1;
		public Integer getId() {
			return id;
		}
		public void setId(Integer 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 int getGender() {
			return gender;
		}
		public void setGender(int gender) {
			this.gender = gender;
		}
		public Date getBirth() {
			return birth;
		}
		public void setBirth(Date birth) {
			this.birth = birth;
		}
		public int getUserType() {
			return userType;
		}
		public void setUserType(int user_type) {
			this.userType = user_type;
		}
		
	}
	
	文章类型类
	public class ArticleClass {
		private Integer id;
		private String name;
		private String desc;
		
		public ArticleClass() {
			
		}
		public ArticleClass(int id) {
			this.id = id;
		}
		public Integer getId() {
			return id;
		}
		public void setId(Integer id) {
			this.id = id;
		}
		public String getName() {
			return name;
		}
		public void setName(String name) {
			this.name = name;
		}
		public String getDesc() {
			return desc;
		}
		public void setDesc(String desc) {
			this.desc = desc;
		}
	}

	文章类

	public class Article {
		private Integer id;
	//	private int classid;
	//	private int userid;
		private String title;
		private String content;
		private ArticleClass articleClass;
		private User user;
		
		public Article() {
			
		}
		
	/*	public int getClassid() {
			return classid;
		}

		public void setClassid(int classid) {
			this.classid = classid;
		}

		public int getUserid() {
			return userid;
		}

		public void setUserid(int userid) {
			this.userid = userid;
		}*/

		public Article(String title, String content) {
			this.title = title;
			this.content = content;
		}
		
		public Integer getId() {
			return id;
		}

		public void setId(Integer id) {
			this.id = id;
		}

		public String getTitle() {
			return title;
		}

		public void setTitle(String title) {
			this.title = title;
		}

		public String getContent() {
			return content;
		}

		public void setContent(String content) {
			this.content = content;
		}

		public ArticleClass getArticleClass() {
			return articleClass;
		}

		public void setArticleClass(ArticleClass articleClass) {
			this.articleClass = articleClass;
		}

		public User getUser() {
			return user;
		}

		public void setUser(User user) {
			this.user = user;
		}

	}

4、解决方案
   问题主要发生在查找文章的时候，如果把文章类型信息和用户信息也查出来并存储在article对象的user和articleClass中
   article映射文件如下
   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE sqlMap
    PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
    "http://java.plugbase.org/dtd/sql-map-2.dtd">
   
   <sqlMap namespace="article">
<typeAlias alias="Article" type="ebiz.sql.article.Article"/>
<typeAlias alias="ArticleClass" type="ebiz.sql.article.ArticleClass"/>
<typeAlias alias="User" type="ebiz.sql.user.User"/>

<resultMap class="Article" id="articleResult">
<result property="id" column="aid"/>
<result property="title" column="title"/>
<result property="content" column="content"/>
<result property="articleClass" select="getArticleClassById" column="classid"/>
<result property="user" select="getUserById" column="userid"/>
</resultMap>

<!-- select sql -->
<select id="getArticleAllCascade" resultMap="articleResult">
select aid,title,content,classid, userid from article
</select>

<select id="getArticleByIdCascade" parameterClass="java.lang.Integer" resultMap="articleResult">
select aid,title,content,classid,userid from article
where article.aid = #id#
</select>

<!-- insert sql -->
<insert id="insertArticle" parameterClass="Article">
insert into article (title, content, classid, userid)
values(#title#, #content#, #articleClass.id#, #user.id#)
</insert>

<!-- update sql -->
<update id="updateArticle" parameterClass="Article">
update article
set title=#title#, content=#content#, classid=#articleClass.id#, userid=#user.id#
where aid=#id#
</update>

<!-- delete sql -->
<delete id="deleteArticleById" parameterClass="java.lang.Integer">
delete from article where aid = #id#
</delete>
  </sqlMap>

  user映射文件

  <?xml version="1.0" encoding="UTF-8" ?>

  <!DOCTYPE sqlMap
    PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
    "http://java.plugbase.org/dtd/sql-map-2.dtd">
  <sqlMap namespace="users">
<typeAlias alias="User" type="ebiz.sql.user.User"/>

<resultMap class="User" id="userResult">
<result property="id" column="id"/>
<result property="username" column="username"/>
<result property="password" column="password"/>
<result property="birth" column="birth"/>
<result property="gender" column="gender"/>
<result property="userType" column="user_type"/>
</resultMap>

<!-- select sql -->
<select id="getUsersAll" resultMap="userResult">
select id, username, password,birth,gender,user_type
from users
order by id desc
</select>

<select id="getUserById" parameterClass="java.lang.Integer" resultMap="userResult">
select id, username, password,birth,gender,user_type
from users
where id=#id#
</select>

<select id="getUserByNameAndPassword" parameterClass="User"  resultClass="java.lang.Integer">
select count(*)
from users
where username=#username# and password=#password#
</select>

<select id="getUserByName" parameterClass="java.lang.String" resultMap="userResult">
select id, username, password,birth,gender,user_type
from users
where username=#username#
</select>

<!-- insert sql -->
<insert id="insertUser" parameterClass="User">
insert into users (username, password,birth,gender,user_type)
values(
#username#, #password#, #birth#, #gender#, #userType#
)
</insert>

<!-- update sql -->
<update id="updateUser" parameterClass="User">
update users
set username=#username#, password=#password#, birth=#birth#, gender=#gender#, user_type=#userType#
where id=#id#
</update>

<!-- delete sql -->
<delete id="deleteUserById" parameterClass="java.lang.Integer">
delete from users where id = #id#
</delete>
    </sqlMap>


  articleClass映射文件

  <?xml version="1.0" encoding="UTF-8"?>
  <!DOCTYPE sqlMap
    PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
    "http://java.plugbase.org/dtd/sql-map-2.dtd">
   
   <sqlMap namespace="article_class">
<typeAlias alias="ArticleClass" type="ebiz.sql.article.ArticleClass"/>


<resultMap class="ArticleClass" id="articleClassResult">
<result property="id" column="acid"/>
<result property="name" column="name"/>
<result property="desc" column="class_desc"/>
</resultMap>

<!-- select sql -->

<select id="getArticleClassById" parameterClass="java.lang.Integer" resultMap="articleClassResult">
select acid, name, class_desc from article_class where acid = #classid#
</select>
   </sqlMap>

5、解释：当使用"getArticleByIdCascade"查询查询文章的时候，查出来之后分别把classid和userid作为参数调用"getUserById"和"getArticleClassById"

只有单向关联。

会出现N+1查询的问题的。