mybatis注解开发 一对多嵌套查询

一个class中有许多student，属于一对多关系。本文利用mybatis注解，嵌套查询方式来实现一对多查询，在根据classId查询class时，顺便查询并展示出class中的student

student实体类

package com.itheima.dao;

public class StudentDao {
    private Integer id;
    private String name;
    private Integer age;
    private Integer cid;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Integer getCid() {
        return cid;
    }

    public void setCid(Integer cid) {
        this.cid = cid;
    }

    @Override
    public String toString() {
        return "StudentDao{" +
                "id=" + id +
                ", name='" + name + ''' +
                ", age=" + age +
                ", cid=" + cid +
                '}';
    }
}

class实体类

package com.itheima.dao;

import java.util.List;

public class ClassDao {
    private Integer id;
    private String className;
    private List<StudentDao> studentsList;
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getClassName() {
        return className;
    }

    public void setClassName(String className) {
        this.className = className;
    }

    @Override
    public String toString() {
        return "ClassDao{" +
                "id=" + id +
                ", className='" + className + ''' +
                ", studentsList=" + studentsList +
                '}';
    }
}

mybatis-config.xml

<?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>
    <!--设置连接数据库的环境-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url"
                          value="jdbc:mysql://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    <!--引入映射文件-->
    <mappers>
        <mapper class="com.itheima.mapper.StudentMapper"/>
        <mapper class="com.itheima.mapper.ClassMapper"/>
    </mappers>

</configuration>

studentMapper.java

package com.itheima.mapper;

import com.itheima.dao.StudentDao;
import org.apache.ibatis.annotations.Many;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface StudentMapper {
    @Select("select * from student where cid = #{cid}")
    @Results({@Result(id = true, property = "id", column = "id"),
              @Result(property = "name",column = "name"),
              @Result(property = "age",column = "age"),
              @Result(property = "cid", column = "cid")})
    List<StudentDao> selectStudentByCid(Integer cid);
}

classMapper.java

package com.itheima.mapper;

import com.itheima.dao.ClassDao;
import org.apache.ibatis.annotations.Many;
import org.apache.ibatis.annotations.Result;
import org.apache.ibatis.annotations.Results;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface ClassMapper {
    @Select("select * from class where id = #{id}")
    @Results({@Result(id =true, property = "id",column = "id"),
              @Result(property = "className",column = "className"),
              @Result(property ="studentsList",column = "id",
                      many = @Many(select = "com.itheima.mapper.StudentMapper.selectStudentByCid"))})
    ClassDao selectClassById(Integer id);
}

testMybatis.java

import com.itheima.dao.ClassDao;
import com.itheima.dao.StudentDao;
import com.itheima.mapper.ClassMapper;
import com.itheima.mapper.StudentMapper;
import org.apache.ibatis.annotations.Mapper;
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 org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class testMybatis {
   @Test
    public void test() throws IOException {
       InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
       SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
       SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(is);
       SqlSession sqlSession = sqlSessionFactory.openSession(true);
       ClassMapper classMapper = sqlSession.getMapper(ClassMapper.class);
       ClassDao res = classMapper.selectClassById(2);
       System.out.println(res.toString());
   }
}

数据库sql文件

INSERT INTO `class` (`id`, `className`) VALUES (1, '软件19-1');
INSERT INTO `class` (`id`, `className`) VALUES (2, '软件19-2');
INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('罗辑', 18, 2, 1);
INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('张三', 18, 2, 2);
INSERT INTO `student` (`name`, `age`, `cid`, `id`) VALUES ('李四', 18, 2, 3);

查询结果可看出，2班的班级信息和2班的学生信息

注意事项：
1、一对多关系中，‘’多‘’方实体类存‘’一‘’的那方的主键作为外键，‘’一‘’的那方的实体类存‘’多‘’的那方的 list<多的那方的实体类属性>以便查询时展示出来。在“一”的那方的mapper接口中定义嵌套查询。

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