mybatis关系映射之一对多和多对一

本实例使用用户（User）和博客（Post）的例子做说明: 一个用户可以有多个博客, 一个博客只对应一个用户

一. 例子(本实体采用maven构建):

1. 代码结构图:

2. 数据库:

t_user 用户表：

CREATE TABLE `t_user` (
  `t_id` int(11) NOT NULL AUTO_INCREMENT,
  `t_name` varchar(26) NOT NULL,
  `t_age` int(11) NOT NULL,
  PRIMARY KEY (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

post 博客表：

CREATE TABLE `t_post` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `url` varchar(255) NOT NULL,
  `uid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `u_id` (`uid`),
  CONSTRAINT `u_id` FOREIGN KEY (`uid`) REFERENCES `t_user` (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

3. 博客实体:

/**
 * 博客类
 */
public class Post {

    private Integer id;
    private String url;
    private Integer uid;

    private User user;

    public User getUser() {
        return user;
    }

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

    public Integer getId() {
        return id;
    }

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

    public String getUrl() {
        return url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public Integer getUid() {
        return uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    @Override
    public String toString() {
        return "Post{" +
                "id=" + id +
                ", url='" + url + '\'' +
                ", uid=" + uid +
                ", user=" + user +
                '}';
    }
}

4. 用户实体:

/**
 * 用户类
 */

public class User {

    private String tName;
    private Integer tAge;
    private Integer tid;
    private List<Post> posts;

    public List<Post> getPosts() {
        return posts;
    }

    public void setPosts(List<Post> posts) {
        this.posts = posts;
    }

    public Integer getTid() {
        return tid;
    }

    public void setTid(Integer tid) {
        this.tid = tid;
    }

    public String gettName() {
        return tName;
    }

    public void settName(String tName) {
        this.tName = tName;
    }

    public Integer gettAge() {
        return tAge;
    }

    public void settAge(Integer tAge) {
        this.tAge = tAge;
    }

    @Override
    public String toString() {
        return "User{" +
                "tName='" + tName + '\'' +
                ", tAge=" + tAge +
                ", tid=" + tid +
                ", posts=" + posts +
                '}';
    }
}

5. 一对多实体配置: UserMapper.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.yiibai.mybatis.models.UserMapper">

    <!-- 根据id查询User, 关联将Post查询出来 -->
    <select id="getUser" resultMap="getUsersMap">
            select * from t_user u,t_post p where u.t_id=p.uid and u.t_id=#{t_id}
        </select>

    <resultMap type="User" id="getUsersMap">
        <id column="t_id" property="tid"></id>
        <result column="t_name" property="tName"/>
        <result column="t_age" property="tAge"/>
        <!-- 一对多的关系 -->
        <!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->
        <collection property="posts" ofType="Post">
            <id column="id" property="id"/>
            <result column="url" property="url"/>
            <result column="uid" property="uid"/>

        </collection>
    </resultMap>

</mapper>

　　

6. 多对一实体配置:PostMapper.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.yiibai.mybatis.models.PostMapper">

    <!-- 根据id查询Post, 关联将User查询出来 -->
    <select id="getPost" resultMap="getPostMap">
            select * from t_user u,t_post p where u.t_id=p.uid and p.id=#{id}
        </select>

    <resultMap type="Post" id="getPostMap">
        <id column="id" property="id"></id>
        <result column="url" property="url"/>
        <result column="uid" property="uid"/>
        <!-- 多对一的关系 -->
        <!-- property: 指的是属性的值, javaType：指的是属性的类型-->
        <association property="user" javaType="User">
            <id column="t_id" property="tid"/>
            <result column="t_name" property="tName"/>
            <result column="t_age" property="tAge"/>

        </association>
    </resultMap>

</mapper>

7. mybatis配置文件: Configure.xml

<?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>

    <properties resource="sql.properties"/>

    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <typeAliases>
        <typeAlias alias="User" type="models.User"/>
        <typeAlias alias="Post" type="models.Post"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driverClassName}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <!-- // power by http://www.yiibai.com -->
        <mapper resource="UserMapper"/>

        <mapper resource="PostMapper"/>
    </mappers>
</configuration>

8. 测试文件:

　　

import models.Post;
import models.User;
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 org.junit.Before;

import java.io.InputStream;

public class Test {

    private SqlSessionFactory ssf;

    @Before
    public void initSF() throws Exception {
        String resource = "Configure";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        ssf = new SqlSessionFactoryBuilder().build(inputStream);
    }

    @org.junit.Test //多对一关联查询
    public void selectPostById() {
        SqlSession session = ssf.openSession();
        User user = session.selectOne("com.yiibai.mybatis.models.UserMapper.getUser", 1);
        System.out.println(user.toString());
        session.close();

    }

    @org.junit.Test //一对多关联查询
    public void selectUserById() {
        SqlSession session = ssf.openSession();
        Post post = session.selectOne("com.yiibai.mybatis.models.PostMapper.getPost", 1);
        System.out.println(post.toString());
        session.close();
    }
}

 9：maven配置

 

  <dependencies>
    <!--dependency>
      <groupId>com.ggband</groupId>
      <artifactId>[the artifact id of the block to be mounted]</artifactId>
      <version>1.0-SNAPSHOT</version>
    </dependency-->

    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>5.1.31</version>
    </dependency>

    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.4.6</version>
    </dependency>

    <dependency>
      <groupId>org.testifyproject.junit4</groupId>
      <artifactId>junit4-core</artifactId>
      <version>1.0.3</version>
    </dependency>

  </dependencies>