多對一處理 和一對多處理的處理

在前面建立的兩個表中，學生和老師的關係，學生對於老師是多對一的關係，老師對於學生是一對多的關係。在MyBatis中處理這樣的相對複雜的關係需要用到物件（association）和 集合（collection）。

多對一

• 實體類

  public class Student2 {
    private int id;
    private String name;
    //學生和老師是多對一的關係，需要在學生中關聯老師
    private Teacher2 teacher;
  }
  public class Teacher2 {
  private int id;
  private String name;
  }

• 介面

  public interface Student2Mapper {
    //獲取所有學生資訊以及他的老師
    List<Student2> getStudent();
    List<Student2> getStudent2();
  }

• xml配置

  <!--按照結果巢狀處理-->
    <select id="getStudent" resultMap="StudentTeacher">
        select s.id sid,s.name sname,t.name tname,t.id tid
        from student s,teacher t
        where s.tid = t.id
    </select>
  
    <resultMap id="StudentTeacher" type="Student2">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher2">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>
  
    <!--通過子查詢的方法-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        select * from student
    </select>
  
    <resultMap id="StudentTeacher2" type="Student2">
        <!-- 複雜的屬性需要單獨處理 物件：association 集合：collection -->
        <association property="teacher" column="tid" javaType="Teacher2" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="Teacher2">
        select * from teacher where id = #{tid}
    </select>

• 一對多

這是從老師的角度，實體類如下

public class Student {
    private int id;
    private String name;
    private int tid;
}

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

List<Teacher> getTeacher2(@Param("tid") int id);

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.hudu.dao.TeacherMapper">
    <select id="getTeacher" resultType="Teacher">
        select * from teacher
    </select>

    <!--按照結果巢狀-->
    <select id="getTeacher2" resultMap="TeacherStudent">
        SELECT s.id sid,s.name sname,t.name tname,t.id tid
        FROM teacher t,student s
        where t.id = s.tid and t.id = #{tid}
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--複雜的屬性，我們需要單獨處理 物件：association 集合：collection
        javaType=""指定屬性的型別
        集合中的泛型資訊，我們使用ofType獲取
        -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

    <select id="getTeacher3" resultMap="TeacherStudent3">
        select * from teacher where id = #{tid}
    </select>

    <resultMap id="TeacherStudent3" type="Teacher">
        <!--<result property="id" column="id"/>-->
        <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>

    <select id="getStudentByTeacherId" resultType="Student">
        select * from student where tid = #{tid};
    </select>
</mapper>

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