MyBatis学习（六）MyBatis关联映射之一对多映射

数据库中一对多通常使用主外键关联，外键应该在多方，即多方维护关系。

下面举一个简单实例来看看MyBatis怎么处理一对多的关系。

1.创建一个项目，导入所需jar包，导入db.properties配置文件，导入log4j.properties配置文件

2.创建一个数据库，在里面创建两张表

-- Table structure for `t_clazz`
-- ----------------------------
DROP TABLE IF EXISTS `t_clazz`;
CREATE TABLE `t_clazz` (
  `id` int(11) NOT NULL,
  `code` varchar(18) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_clazz
-- ----------------------------
INSERT INTO `t_clazz` VALUES ('1', '一班');
INSERT INTO `t_clazz` VALUES ('2', '二班');

-- Table structure for `t_student`
-- ----------------------------
DROP TABLE IF EXISTS `t_student`;
CREATE TABLE `t_student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(18) NOT NULL,
  `sex` varchar(3) NOT NULL,
  `age` int(11) NOT NULL,
  `cid` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `cid` (`cid`),
  CONSTRAINT `cid` FOREIGN KEY (`cid`) REFERENCES `t_clazz` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t_student
-- ----------------------------
INSERT INTO `t_student` VALUES ('1', '张三', '男', '11', '1');
INSERT INTO `t_student` VALUES ('2', '李四', '男', '12', '2');
INSERT INTO `t_student` VALUES ('3', '小红', '女', '13', '1');

3.编写对应的实体类

public class Student {

    private Integer id;
    private String name;
    private String sex;
    private Integer age;
    //关联的clazz对象
    private Clazz clazz;

public class Clazz {

    private Integer id;
    private String code;
    //关联的student集合
    private List<Student> students;

4.编写对应的SQL映射文件

ClazzMapper.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.dj.mapper.ClazzMapper">

    <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
        select * from t_clazz where id =#{id}
    </select>
    <resultMap type="com.dj.domain.Clazz" id="clazzResultMap">
        <id property="id" column="id"/>
        <resultproperty="code" column="code"/>
        <!--property表示返回类型Clazz的属性students
            column表示将id作为参数进行之后的查询
            fetchtype表示懒加载
            javaType表示属性对应的类型 
            ofType表示集合当中的类型
        -->
        <collection property="students" column="id" fetchType="lazy" 
                javaType="ArrayList" ofType="com.dj.domain.Student" 
                select="com.dj.mapper.StudentMapper.selectStudentByClazzId">
                <id property="id" column="id"/>
                <resultproperty="name" column="name"/>
                <resultproperty="sex" column="sex"/>
                <resultproperty="age" column="age"/>
                </collection>
    </resultMap>
    
    
</mapper>

StudentMapper.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">

<!-- namespace指用户自定义的命名空间 -->
<mapper namespace="com.dj.mapper.StudentMapper">

    <select id="selectStudentByClazzId" parameterType="int" resultType="com.dj.domain.Student">
        select * from t_student where cid=#{id}
    </select>
</mapper>

5.编写mybatis-config.xml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>
     <!-- 引入 外部db.properties文件-->
     <properties resource="db.properties"/>
     <!-- 指定 MyBatis 所用日志的具体实现-->
     <settings>
         <setting name="logImpl" value="log4j"/>
     </settings>
     <!-- 环境配置 -->
     <environments default="mysql">
         <environment id="mysql">
             <!-- 指定事务类型 -->
             <transactionManager type="JDBC"/>
                 <!--  dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现。 -->
             <dataSource type="POOLED">
                 <property name="driver" value="${driver}"/>
                 <property name="url" value="${url}"/>
                 <property name="username" value="${username}"/>
                 <property name="password" value="${password}"/>
             </dataSource>
         </environment>
     </environments>
     <!-- SQL映射文件位置 -->
     <mappers>
         <mapper resource="com/dj/mapper/StudentMapper.xml"/>
         <mapper resource="com/dj/mapper/ClazzMapper.xml"/>
     </mappers>
 </configuration>

6.mybatis建议通过mapper接口的代理对象访问mybatis，该对象关联了一个sqlsession对象，开发者可以通过该对象直接调用方法操作数据库。

注意: mapper接口对象的类名必须和之前的mapper.xml的namespace一致，方法名和参数名及返回类型也要与mapper.xml的配置一致。

public interface ClazzMapper {
    //根据id查询班级信息
    Clazz selectClazzById(int id);
    
}

7.测试

public class OneToManyTest {
    
    public static void main(String[] args) throws Exception {
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //获得mapper接口的代理对象
        ClazzMapper mapper = sqlSession.getMapper(ClazzMapper.class);
        //调用接口中的方法
        Clazz clazz = mapper.selectClazzById(1);
        List<Student> students = clazz.getStudents();
        for (Student student : students) {
            System.out.println(student);
        }
        
    }

}

在控制台可以看到如下结果：

测试成功。