Mybatis多对一的处理

Mybatis多对一的处理

多个学生对应一个老师

对于学生来说：关联一个老师 对象
1.建表：

注意两张表之间的主外键的关系

2.测试环境的搭建

• 导包
• mybatis核心配置文件
• mybatis工具类
• 实体类
• dao层接口
• mapper配置文件
• 测试

3.导包

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>mybatisA</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>mybatis1</module>
        <module>mybatis2</module>
        <module>mybatis3</module>
        <module>mybatis4</module>
    </modules>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.7</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.27</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.12</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

4.mybatis核心配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>
    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql:///mybatis?serverTimezone=UTC"/>
                <property name="username" value="root"/>
                <property name="password" value="1234"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="com/momo/dao/UserMapper.xml"/>
    </mappers>
</configuration>

5.实体类

package com.momo.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int tid;
    private String name;
}

package com.momo.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.Date;


@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int sid;
    private String name;
    private int age;
    private String sex;
    private Date birthday;
    // 作为学生来讲，要关联一个老师。
    private Teacher teacher;
}

6.dao层接口的编写

package com.momo.dao;

import com.momo.entity.Student;

import java.util.List;

public interface UserMapper {
    //查询所有
    public List<Student> findByAllStudent1();
}

7.mapper文件的编写

<?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.momo.dao.UserMapper">
<!--    按照结果嵌套查询-->
    <select id="findByAllStudent1" resultMap="s_t">
        select s.sid,s.name,s.age,s.sex,s.birthday,t.tid,t.name from teacher t,student s where t.tid = s.tid;
    </select>
    <resultMap id="s_t" type="com.momo.entity.Student">
        <id property="sid" column="sid"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="sex" column="sex"/>
        <result property="birthday" column="birthday"/>
        <association property="teacher" javaType="com.momo.entity.Teacher">
            <id property="tid" column="tid"/>
            <result property="name" column="name"/>
        </association>
    </resultMap>


<!--    根据查询嵌套查询-->
    <select id="findByAllStudent1" resultMap="s_t">
        select * from student;
    </select>
    <resultMap id="s_t" type="com.momo.entity.Student">
        <id property="sid" column="sid"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <result property="sex" column="sex"/>
        <result property="birthday" column="birthday"/>
        <association property="teacher" column="tid" javaType="com.momo.entity.Teacher" select="findTeacher"/>
    </resultMap>
    <select id="findTeacher" resultType="com.momo.entity.Teacher" parameterType="int">
        select * from teacher where tid = #{tid};
    </select>
</mapper>

8.在mybatis核心配置文件中记得绑定mapper配置文件

9.测试

package com.momo.test;

import com.momo.dao.UserMapper;
import com.momo.entity.Student;
import com.momo.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.After;
import org.junit.Before;

import java.util.List;

public class Test {
    private SqlSession sqlSession;
    private UserMapper userMapper;

    @Before
    public void init(){
        sqlSession = MybatisUtils.getSqlSession();
        userMapper = sqlSession.getMapper(UserMapper.class);
    }

    @After
    public void close(){
        sqlSession.close();
    }

    @org.junit.Test
    public void testFindByAllStudent1(){
        List<Student> list = userMapper.findByAllStudent1();
        list.forEach(System.out::println);
    }
}

注意：

• mybatis核心配置文件中一定要绑定mapper配置文件
• mapper配置文件中mapper中一定要加命名空间namespace
• 复杂属性在mapper中配置文件对象要用association
• 复杂属性在mapper中配置文件集合要用collection
• 注意属性名与字段名的问题