MyBatis简易教程（03）：mybatis关联映射（一对一）

MyBatis简易教程汇总，详见：https://www.cnblogs.com/uncleyong/p/17984096

准备测试数据

country表

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for country
-- ----------------------------
DROP TABLE IF EXISTS `country`;
CREATE TABLE `country` (
  `c_id` int(11) NOT NULL AUTO_INCREMENT,
  `c_name` varchar(255) DEFAULT NULL,
  `president_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`c_id`),
  UNIQUE KEY `unique_president_id` (`president_id`) USING BTREE,
  CONSTRAINT `fk_president_id` FOREIGN KEY (`president_id`) REFERENCES `president` (`p_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of country
-- ----------------------------
INSERT INTO `country` VALUES ('1', '美国', '1');
INSERT INTO `country` VALUES ('2', '巴西', '2');

president表

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for president
-- ----------------------------
DROP TABLE IF EXISTS `president`;
CREATE TABLE `president` (
  `p_id` int(11) NOT NULL AUTO_INCREMENT,
  `p_name` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`p_id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of president
-- ----------------------------
INSERT INTO `president` VALUES ('1', '拜登');
INSERT INTO `president` VALUES ('2', '卢拉');

模型

示例

通过国家id查国家及总统

Country类添加President属性（说明：下面presidentId这个属性可以省略，因为和对象属性president里面的p_id一样）

package com.qzcsbj.bean;


public class Country {

  private long cId;
  private String cName;
  private long presidentId;
  private President president;


  public long getCId() {
    return cId;
  }

  public void setCId(long cId) {
    this.cId = cId;
  }


  public String getCName() {
    return cName;
  }

  public void setCName(String cName) {
    this.cName = cName;
  }


  public long getPresidentId() {
    return presidentId;
  }

  public void setPresidentId(long presidentId) {
    this.presidentId = presidentId;
  }

  public President getPresident() {
    return president;
  }

  public void setPresident(President president) {
    this.president = president;
  }

  @Override
  public String toString() {
    return "Country{" +
            "cId=" + cId +
            ", cName='" + cName + '\'' +
            '}';
  }
}

President类

package com.qzcsbj.bean;


public class President {

  private long pId;
  private String pName;


  public long getPId() {
    return pId;
  }

  public void setPId(long pId) {
    this.pId = pId;
  }


  public String getPName() {
    return pName;
  }

  public void setPName(String pName) {
    this.pName = pName;
  }

  @Override
  public String toString() {
    return "President{" +
            "pId=" + pId +
            ", pName='" + pName + '\'' +
            '}';
  }
}

mapper接口

package com.qzcsbj.mapper;

import com.qzcsbj.bean.Country;

/**
 * @公众号 : 全栈测试笔记
 * @博客 : www.cnblogs.com/uncleyong
 * @微信 : ren168632201
 * @描述 : <>
 */
public interface CountryMapper {
    // 根据id查国家，并查询其对应的总统
    public Country getCountryById(int cId);
}

映射文件：CountryMapper.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.qzcsbj.mapper.CountryMapper">
    <select id="getCountryById" resultMap="CountryMap">
        select * from country c join president p on c.president_id = p.p_id
        where c.c_id=#{cId}
    </select>

    <resultMap id="CountryMap" type="Country">
        <id column="c_id" property="cId"/>
        <result column="c_name" property="cName"/>
        <result column="president_id" property="presidentId"/>
        <association property="president" javaType="President">
            <id column="p_id" property="pId"/>
            <result column="p_name" property="pName"/>
        </association>
    </resultMap>

</mapper>

测试类

package com.qzcsbj;

import com.qzcsbj.bean.*;
import com.qzcsbj.mapper.*;
import com.qzcsbj.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.util.List;


/**
 * @公众号 : 全栈测试笔记
 * @博客 : www.cnblogs.com/uncleyong
 * @微信 : ren168632201
 * @描述 : <>
 */
public class testMybatis {

    SqlSession session = null;
    CountryMapper countryMapper = null;

    @Before
    public void init(){
        System.out.println("初始化。。。");
        session = MyBatisUtils.getSession();
        countryMapper = session.getMapper(CountryMapper.class);
    }

    @After
    public void destory(){
        System.out.println("关闭session");
        MyBatisUtils.closeSession(session);
    }


    @Test
    public void testGetCountryById(){
        Country country = countryMapper.getCountryById(1);
        System.out.println("国家信息是：" + country);
        President president = country.getPresident();
        System.out.println("总统信息是：" + president);
    }
}

结果

通过总统id查总统及国家

President类下加Country属性

package com.qzcsbj.bean;


public class President {

  private long pId;
  private String pName;
  private Country country;


  public long getPId() {
    return pId;
  }

  public void setPId(long pId) {
    this.pId = pId;
  }


  public String getPName() {
    return pName;
  }

  public void setPName(String pName) {
    this.pName = pName;
  }

  public Country getCountry() {
    return country;
  }

  public void setCountry(Country country) {
    this.country = country;
  }

  @Override
  public String toString() {
    return "President{" +
            "pId=" + pId +
            ", pName='" + pName + '\'' +
            '}';
  }
}

Country类

package com.qzcsbj.bean;


public class Country {

  private long cId;
  private String cName;
  private long presidentId;
  private President president;


  public long getCId() {
    return cId;
  }

  public void setCId(long cId) {
    this.cId = cId;
  }


  public String getCName() {
    return cName;
  }

  public void setCName(String cName) {
    this.cName = cName;
  }


  public long getPresidentId() {
    return presidentId;
  }

  public void setPresidentId(long presidentId) {
    this.presidentId = presidentId;
  }

  public President getPresident() {
    return president;
  }

  public void setPresident(President president) {
    this.president = president;
  }

  @Override
  public String toString() {
    return "Country{" +
            "cId=" + cId +
            ", cName='" + cName + '\'' +
            '}';
  }
}

mapper接口

package com.qzcsbj.mapper;

import com.qzcsbj.bean.Country;
import com.qzcsbj.bean.President;

/**
 * @公众号 : 全栈测试笔记
 * @博客 : www.cnblogs.com/uncleyong
 * @微信 : ren168632201
 * @描述 : <>
 */
public interface PresidentMapper {
    // 根据id查总统，并查询其对应的国家
    public President getPresidentById(int pId);
}

映射文件：PresidentMapper.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.qzcsbj.mapper.PresidentMapper">
    <select id="getPresidentById" resultMap="PresidentMap">
        select * from country c join president p on c.president_id = p.p_id
        where p.p_id=#{pId}
    </select>

    <resultMap id="PresidentMap" type="President">
        <id column="p_id" property="pId"/>
        <result column="p_name" property="pName"/>
        <association property="country" javaType="Country">
            <id column="c_id" property="cId"/>
            <result column="c_name" property="cName"/>
            <result column="president_id" property="presidentId"/>
        </association>
    </resultMap>

</mapper>

测试类

package com.qzcsbj;

import com.qzcsbj.bean.*;
import com.qzcsbj.mapper.*;
import com.qzcsbj.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import java.util.List;


/**
 * @公众号 : 全栈测试笔记
 * @博客 : www.cnblogs.com/uncleyong
 * @微信 : ren168632201
 * @描述 : <>
 */
public class testMybatis {

    SqlSession session = null;
    PresidentMapper presidentMapper = null;

    @Before
    public void init(){
        System.out.println("初始化。。。");
        session = MyBatisUtils.getSession();
        presidentMapper = session.getMapper(PresidentMapper.class);
    }

    @After
    public void destory(){
        System.out.println("关闭session");
        MyBatisUtils.closeSession(session);
    }


    @Test
    public void testGetPresidentById(){
        President president = presidentMapper.getPresidentById(1);
        System.out.println("总统信息是：" + president);
        Country country = president.getCountry();
        System.out.println("国家信息是：" + country);
    }
}

结果