Mybatis多對一以及一對多關聯映射

前期準備：

• 建表：

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

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

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, '張珊', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (2, '張思', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (3, '張武', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (4, '張柳', 1);
INSERT INTO student (`id`, `name`, `tid`) VALUES (5, '張琪', 1);
           

• 建立實體類

Teacher：

package com.pojo;

import java.util.List;

public class Teacher {
   private int id;
   private String name;
   //一個老師有多個學生
   private List<Student> studentList;
   public Teacher() {
   }

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

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

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

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

Student：

package com.pojo;

public class Student {
    private int id;
    private String name;
    //學生需要關聯一個老師
    private Teacher teacher;

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

    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;
    }

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }

    public Student() {
    }
}
           

多對一：多個學生對應一個老師

• 建立接口StudentMapper

package com.dao;

import com.pojo.Student;
import org.apache.ibatis.annotations.Param;
public interface StudentMapper {
    Student getStudent(@Param("id")int id);
}
           

• 建立mapper映射檔案StudentMaper.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.dao.StudentMapper">
    <select id="getStudent" parameterType="int" resultMap="getStudentMap">
        select s.id sid,s.name sname,t.id as tid,t.name tname
        from student s,teacher t
        where s.id=#{id} and s.tid=t.id;
    </select>
    <resultMap id="getStudentMap" type="Student">
        <result property="id" column="sid"></result>
        <result property="name" column="sname"></result>
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"></result>
            <result property="name" column="tname"></result>
        </association>
    </resultMap>
</mapper>
           

說明：

property 映射到列結果的字段或屬性。

column 資料庫中的列名，或者是列的别名。

一對多：一個老師對應多個學生

• 建立接口TeacherMapper

package com.dao;

import com.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
public interface TeacherMapper {
    Teacher getTeacher(@Param("id")int id);
}
           

• 建立mapper映射檔案TeacherMaper.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.dao.TeacherMapper">
    <select id="getTeacher" parameterType="int" resultMap="getTeacherMap">
        select s.id sid,s.name sname,t.name tname,t.id tid
        from student s,teacher t
        where s.tid=t.id and t.id=#{id};
    </select>
    <resultMap id="getTeacherMap" type="com.pojo.Teacher">
        <result property="id" column="tid"></result>
        <result property="name" column="tname"></result>
        <!--集合中的泛型資訊，我們用oftype擷取-->
        <collection property="studentList" ofType="com.pojo.Student">
            <result property="id" column="sid"></result>
            <result property="name" column="sname"></result>
        </collection>
    </resultMap>
</mapper>
           

小結

1. 關聯 - association 多對一
2. 集合 - collection 一對多
3. javaType & ofType
   1. JavaType用來指定實體中屬性類型
   2. ofType映射到list中的類型，泛型中的限制類型