mybatis的学习7___一对多的实现

（本案列为maven项目下进行：）

1.实验环境的搭建：

（一张学生表，一张老师表）

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO teacher(`id`, `name`) VALUES (1, '秦老师'); 

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8


INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '武同学', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '李同学', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '武同学', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '姚同学', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '夏同学', '1');

需求：通过老师的编号，查找到，该老师下面的所有学生信息(id，name)：

2.老师pojo类的编写：

由于一个老师对应着多个学生，所以老师类中存在属性：List<Student> studentList

public class Teacher {
    private int id;
    private String name;
    private List<Student> studentList;

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> studentList) {
        this.id = id;
        this.name = name;
        this.studentList = studentList;
    }

    public List<Student> getStudentList() {
        return studentList;
    }

    public void setStudentList(List<Student> studentList) {
        this.studentList = studentList;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", studentList=" + studentList +
                '}';
    }
}

3.Dao层TeacherMapper接口的编写：

public interface TeacherDao {


    //获取老师和老师下学生信息
    Teacher getInfo(int id);

    //获取老师和老师下学生信息
    Teacher getTeacherTwo(int id);

}

4.接口的xml配置文件的编写teacherMapper.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.xbf.dao.TeacherDao">

    <!--面向对象的处理方式-->
    <!--使用 collection标签-->
    <select id="getInfo" resultMap="T">
        select s.id as sid,s.name as sname,t.name as tname from student s,teacher t where t.id=s.tid and t.id=#{id}
    </select>

    <resultMap id="T" type="Teacher">
        <result column="tname" property="name"/>
        <collection property="studentList" ofType="Student">
            <result column="sid" property="id"/>
            <result column="sname" property="name"/>
        </collection>
    </resultMap>


    <!--数据库的方式-->
    <select id="getTeacherTwo" resultMap="TeacherStudent2">
        select * from mybatis.teacher where id = #{id}
    </select>
    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="studentList" javaType="ArrayList" ofType="Student" column="id" select="T2"/>
    </resultMap>
    <select id="T2" resultType="Student">
        select * from mybatis.student where tid = #{id}
    </select>

</mapper>

5.mybatis-config.xml的映射器：

  <mappers>
       <mapper resource="com/xbf/dao/teacherMapper.xml"></mapper>
    </mappers>

6.测试类的编写：

public class StudentDaoTest {

    @Test
    public void getInfoTest(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherDao mapper = sqlSession.getMapper(TeacherDao.class);
        Teacher teacher = mapper.getInfo(1);
        System.out.println(teacher);
    }


    @Test
    public void getTeacherTwo(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherDao mapper = sqlSession.getMapper(TeacherDao.class);
        Teacher teacher = mapper.getTeacherTwo(1);

        System.out.println(teacher.getName());
        System.out.println(teacher.getStudentList());

    }
}

总结：

一对多使用: collection

项目第一次跑的时候报错：

org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): com.xbf.dao.Teacher

后来仔细检查发现，在maven导出的target目录中找不到 teacherMapper.xml，因为src/java中的.xml或.properties资源配置文件没有导入。

在maven的pom.xml中添加如下代码后，在target文件中找到了 teacherMapper.xml，成功运行！

 <build>
        <!--希望maven在导出项目的时候(target),能够将我们的配置及资源导出-->
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
        </resources>

    </build>