MyBatis一对多映射实现方式

曾经终败给现在 2022-07-13 03:09 163阅读 0赞

## **Mybatis映射**\#\# ##

一、一对多  
一个班级有多个学生，但是一个学生只有一个班级，学生就是多的那方。学生只有一个班级属性，班级有多个学生属性。所以学生类中只需要一个班级类的实例，班级里面需要一个列表来存放学生属性。  
先在学生类中加入班级属性：

public class Student {
        private int id;
        private String name;
        private int classId;
    
        private Clazz clazz;

在映射文件中加入映射语句：

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
        <mapper namespace="com.ffy.mapping.studentMapper">
            <resultMap type="com.ffy.POJO.Student" id="StudentResultMap">
                <id property="id" column="id" jdbcType="INTEGER"/>
                <result property="name" column="name" jdbcType="VARCHAR"/>
                <result property="classId" column="class_id" jdbcType="INTEGER"/>
                
                <association property="clazz" column="class_id" javaType="com.ffy.POJO.Clazz" select="com.ffy.mapping.classMapper.getClassById" />
            </resultMap>
            <select id="getAllStudents" resultMap="StudentResultMap">
                 select * from student
            </select>
            <select id="selectStudentInClass" parameterType="int" resultMap="StudentResultMap">
                select * from student where class_id=#{id};
            </select>
        </mapper>

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.ffy.mapping.classMapper">
        <resultMap type="com.ffy.POJO.Clazz" id="ClassResultMap">
            <id column="id" property="id" jdbcType="INTEGER" />
            <result property="name" column="name" jdbcType="VARCHAR" />
            <collection property="students" column="id" javaType="ArrayList" ofType="com.ffy.POJO.Student" select="com.ffy.mapping.studentMapper.selectStudentInClass"></collection>
        </resultMap>
        
        <select id="getClassById" resultMap="ClassResultMap" parameterType="int">
            select * from class where id=#{id}
        </select>
        <select id="selectAllClass" resultMap="ClassResultMap">
            select * from class
        </select>
    </mapper>

一的那方：  
在Clazz中加入一个列表，用于存放多个学生：

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

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    <mapper namespace="com.ffy.mapping.classSafeMapper">
        <resultMap type="com.ffy.POJO.Clazz" id="ClassResultMap">
            <id column="id" property="id" jdbcType="INTEGER" />
            <result property="name" column="name" jdbcType="VARCHAR" />
            
            <collection property="students" column="id" javaType="ArrayList" ofType="com.ffy.POJO.Student" select="com.ffy.mapping.studentSafeMapper.SafeselectStudentInClass"></collection>
        </resultMap>
        <select id="getClassById" resultMap="ClassResultMap" parameterType="int">
            select * from class where id=#{id}
        </select>
        <select id="selectAllClass" resultMap="ClassResultMap">
            select * from class
        </select>
    
    </mapper>

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Config3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
        <mapper namespace="com.ffy.mapping.studentSafeMapper">
            <resultMap type="com.ffy.POJO.Student" id="SafeStudentResultMap">
                <id property="id" column="id" jdbcType="INTEGER"/>
                <result property="name" column="name" jdbcType="VARCHAR"/>
                <result property="classId" column="class_id" jdbcType="INTEGER"/>
            </resultMap>
            <select id="getAllStudents" resultMap="SafeStudentResultMap">
                 select * from student
            </select>
            
            <select id="SafeselectStudentInClass" parameterType="int" resultMap="SafeStudentResultMap">
                select * from student where class_id=#{id};
            </select>
        </mapper>