Mybatis第五讲——关联一对多

Mybatis第四讲——关联一对一（2）

 目录结构：

数据表：

t_grade:

t_student:

t_address:

主外键关联关系：

Grade 实体类：

package com.java1234.model;

import java.util.List;

public class Grade {

	private Integer id;
	private String gradeName;
	private List<Student> students;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getGradeName() {
		return gradeName;
	}
	public void setGradeName(String gradeName) {
		this.gradeName = gradeName;
	}
	public List<Student> getStudents() {
		return students;
	}
	public void setStudents(List<Student> students) {
		this.students = students;
	}
	@Override
	public String toString() {
		return "Grade [id=" + id + ", gradeName=" + gradeName + ", students=" + students + "]";
	}
	
	
}

GradeMapper.java：

package com.java1234.mappers;

import com.java1234.model.Grade;

public interface GradeMapper {
	public Grade findGradeById(Integer id);
}

GradeMapper.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.java1234.mappers.GradeMapper">
	
	<resultMap type="Grade" id="gradeResult">
	 	<result property="id" column="id"/>
		<result property="gradeName" column="gradeName"/>
		<collection property="students" column="id" select="com.java1234.mappers.StudentMapper.getStudentByGradeId"></collection>
	</resultMap>
	
<select id="findGradeById" resultMap="gradeResult">
select * from t_grade where id=#{id}
</select>
</mapper> 

 StudentMapper.java：

与上一讲相比，添加了getStudentByGradeId()函数接口。

StudentMapper.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.java1234.mappers.StudentMapper">

	<insert id="add" parameterType="Student"  >
		insert into t_student values(null,#{name},#{age})
	</insert>
	
	<delete id="delete" parameterType="Integer">
	 delete from t_student where id=#{id}
	</delete>
	
	<update id="update" parameterType="Student">
	update t_student set name=#{name},age=#{age} where id=#{id}
	</update>
	
	<select id="getStudentById" parameterType="Integer" resultType="Student">
	select * from t_student where id=#{id}
	</select>

    <select id="getStudentByGradeId" parameterType="Integer" resultType="Student">
	select * from t_student where gradeId=#{gradeId}
	</select>
	
	<resultMap type="Student" id="StudentResult">
		<id property="id" column="id"/>
		<result property="name" column="name"/>
		<result property="age" column="age"/>
		
		<association property="address" column="addressId" select="com.java1234.mappers.AddressMapper.findAddressById"/>
	</resultMap> 
	
	<select id="getAllStudents" resultMap="StudentResult">
	select * from t_student 
	</select>
	
	<select id="findStudentWithAddress" resultMap="StudentResult" parameterType="Integer">
	select * from t_student t1,t_address t2 where t1.addressId=t2.id and t1.id=#{id} 
	</select>
</mapper> 

主要是添加了getStudentByGradeId函数的SQL实现：

 

测试类：

package com.java1234.service;

import org.apache.ibatis.session.SqlSession;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;

import com.java1234.mappers.GradeMapper;
import com.java1234.model.Grade;
import com.java1234.util.SqlSessionFactoryUtil;

public class GradeTest {
	private SqlSession sqlSession=null;
	private GradeMapper gradeMapper=null;
	@BeforeEach
	void setUp() throws Exception {
		sqlSession=SqlSessionFactoryUtil.openSession();
		gradeMapper=sqlSession.getMapper(GradeMapper.class);
	}

	@AfterEach
	void tearDown() throws Exception {
		sqlSession.close();
	}
	

	/**
	 * 通过gradeId获取学生信息
	 */
	@Test
	void getStudentByGradeId() {
		Grade grade=gradeMapper.findGradeById(1);
		System.out.println(grade);
	}
}

输出：

Grade [id=1, gradeName=大学一年级, students=[Student [id=1, name=张三, age=22, address=null], Student [id=4, name=王五, age=33, address=null]]]

这里address为空，有点问题，不过和我们这讲的一对多无关，暂且放下，后续再解决。