MyBatis友人帐之一对多和多对一处理
一、概念
多对一的理解：
-
多个学生对应一个老师
-
如果对于学生这边，就是一个多对一的现象，即从学生这边关联一个老师！
1.1数据库设计
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', '小明', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');1.2idea配置
1.3其他配置
数据库配置文件
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/test?useSSL=false&useUnicode=true&characterEncoding=utf8&serverTimezone=GMT
username=root
password=b123456日志文件
#????DEBUG????????console?file???????console?file?????????
log4j.rootLogger=DEBUG,console,file
#??????????
log4j.appender.console = org.apache.log4j.ConsoleAppender
log4j.appender.console.Target = System.out
log4j.appender.console.Threshold=DEBUG
log4j.appender.console.layout = org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n
#?????????
log4j.appender.file = org.apache.log4j.RollingFileAppender
log4j.appender.file.File=./log/kuang.log
log4j.appender.file.MaxFileSize=10mb
log4j.appender.file.Threshold=DEBUG
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n
#??????
log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG1.4编写实体类
package com.yanyu.pojo;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
//多个学生可以是同一个老师，即多对一
private Teacher teacher;
}package com.yanyu.pojo;
import lombok.Data;
@Data //GET,SET,ToString，有参，无参构造
public class Teacher {
private int id;
private String name;
}1.5编写接口
package com.yanyu.dao;
public interface StudentMapper {
}package com.yanyu.dao;
import com.yanyu.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
public interface TeacherMapper {
@Select("select * from teacher where id = #{tid}")
Teacher getTeacher (@Param("tid") int id);
}1.6编写实体类配置
1.7控制
package com.yanyu.utils;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.InputStream;
public class MybatisUtils {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
//获取SqlSession连接
// public static SqlSession getSession(){
// return sqlSessionFactory.openSession();
// }
//获取SqlSession连接
public static SqlSession getSession(){
return getSession(true); //事务自动提交
}
public static SqlSession getSession(boolean flag){
return sqlSessionFactory.openSession(flag);
}
}二、多对一的需求
2.1按查询嵌套处理
给StudentMapper接口增加方法
//获取所有学生及对应老师的信息
public List getStudents(); 编写对应的Mapper文件
：定义了一个StudentTeacher结果映射，它将数据库查询的结果映射到Student类上，并且Student类中有一个复杂类型的属性teacher，它需要传入两个参数id和name，它们对应数据库中的tid列，并且teacher属性值是通过调用getTeacher这个SQL语句来获取的。
测试
@Test
public void testGetStudents(){
SqlSession session = MybatisUtils.getSession();
StudentMapper mapper = session.getMapper(StudentMapper.class);
List students = mapper.getStudents();
for (Student student : students){
System.out.println(
"学生名:"+ student.getName()
+"\t老师:"+student.getTeacher().getName());
}
} 2.2按结果嵌套处理
接口方法编写
public List getStudents2(); 编写对应的mapper文件
测试
@Test
public void testGetTeacher2(){
SqlSession session = MybatisUtils.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher2(1);
System.out.println(teacher.getName());
System.out.println(teacher.getStudents());
}2.3小结
按照查询进行嵌套处理就像SQL中的子查询
按照结果进行嵌套处理就像SQL中的联表查询
三、 一对多处理
一对多的理解：
一个老师拥有多个学生
如果对于老师这边，就是一个一对多的现象，即从一个老师下面拥有一群学生（集合）！
3.1环境配置
@Data
public class Student {
private int id;
private String name;
private int tid;
}@Data
public class Teacher {
private int id;
private String name;
//一个老师多个学生
private List students;
} 3.2按结果嵌套处理
TeacherMapper接口编写方法
//获取指定老师，及老师下的所有学生
public Teacher getTeacher(int id);编写接口对应的Mapper配置文件
测试
@Test
public void testGetTeacher(){
SqlSession session = MybatisUtils.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher.getName());
System.out.println(teacher.getStudents());
}3.3按查询嵌套处理
TeacherMapper接口编写方法
public Teacher getTeacher2(int id);编写接口对应的Mapper配置文件
测试
@Test
public void testGetTeacher2(){
SqlSession session = MybatisUtils.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher2(1);
System.out.println(teacher.getName());
System.out.println(teacher.getStudents());
}3.4小结
1、关联-association
2、集合-collection
3、所以association是用于一对一和多对一，而collection是用于一对多的关系
4、JavaType和ofType都是用来指定对象类型的
JavaType是用来指定pojo中属性的类型
ofType指定的是映射到list集合属性中pojo的类型。