Mybatis第五讲——关联一对多

Love The Way You Lie 2022-03-16 00:46 195阅读 0赞

[Mybatis第四讲——关联一对一（2）][Mybatis_2]

目录结构：

![20190225194028853.png][]

数据表：

t\_grade:

![20190225195413945.png][]

t\_student:

![20190225195449809.png][]

t\_address:

![2019022519553975.png][]

主外键关联关系：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70][]

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：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70 1][]

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：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70 2][]

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。

[Mybatis_2]: https://blog.csdn.net/yongbutingxide/article/details/87913563
[20190225194028853.png]: /images/20220316/a1e198481f5b49aeaad4eeb2585d4004.png
[20190225195413945.png]: /images/20220316/ca4185301bfc4abcb808e98212120511.png
[20190225195449809.png]: /images/20220316/e28dcf2cc6934100bd5456a731de4171.png
[2019022519553975.png]: /images/20220316/60a524f7529944ad86da25e785a7c162.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70]: /images/20220316/5320bd14888548fa88ceae474c580c45.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70 1]: /images/20220316/6e119b5eeb8c417783798b818edbf6d6.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lvbmdidXRpbmd4aWRl_size_16_color_FFFFFF_t_70 2]: /images/20220316/e558e1a6d41c45bfb64ec6032c0b72b2.png