Mybatis 多表实现多对多查询、添加操作

Mybatis 多表实现多对多查询、添加操作

• 学习内容：
• • 1. 多对多之添加操作
  • • 1.1、需求
    • 1.2、数据库表(多对多数据库设计需要设计中间表)
    • 1.3、类设计
    • 1.4、Mapper 接口和 Mapper XML 文件编写
    • 1.5、编写单元测试类
  • 2. 单向多对多之额外 SQL 查询
  • • 2.1、需求
    • 2.2、需要添加 Mapper 接口 和 Mapper XML
    • 2.3、单元测试方法
• 总结：

学习内容：

---

1. 多对多之添加操作

1.1、需求

保存两个学生和两个老师，且这两个老师都教了这个两个学生。

1.2、数据库表(多对多数据库设计需要设计中间表)

• 表设计
  

1.3、类设计

• Teacher 实体类

package com.yy.domain;
@Setter
@Getter
@ToString
public class Teacher {
	private Long id;
	private String name;
}

• Employee 实体类

package com.yy.domain;
@Setter
@Getter
@ToString
public class Student {
private Long id;
private String name;
	// 关联属性
	private List<Teacher> teachers = new ArrayList<>();
}

1.4、Mapper 接口和 Mapper XML 文件编写

注意: Mapper XML 放置的位置是在 resources 目录下。

• TeacherMapper 接口

package com.yy.mapper;
public interface TeacherMapper {
	void save(Teacher teacher);
}

• TeacherMapper.xml 文件

<!--
	useGeneratedKeys=true 获取数据库保存数据的的主键值
	keyProperty="id" 主键设置设置对象的 id 属性
-->
<insert id="save" useGeneratedKeys="true" keyProperty="id">
	INSERT INTO teacher(name) VALUES(#{name})
</insert>

• StudentMapper 接口

package com.yy.mapper;
public interface StudentMapper {
	void save(Student student);
	// 往中间表插入关系数据
	void insertRelation(@Param("teacherId")Long teacherId, @Param("studentId")Long studentId);
}

• StudentMapper.xml 文件

<!--
	useGeneratedKeys=true 获取数据库保存数据的的主键值
	keyProperty="id" 主键设置设置对象的 id 属性
-->
<insert id="save" useGeneratedKeys="true" keyProperty="id">
	INSERT INTO student(name) VALUES(#{name})
</insert>
<insert id="insertRelation">
	INSERT INTO teacher_student(teacher_id, student_id) VALUES (#{teacherId}, # {studentId})
</insert>

1.5、编写单元测试类

public class Many2manyTest {
        @Test
        public void testSave() throws Exception {
            Teacher teacher1 = new Teacher();
            teacher1.setName("波老师");
            Teacher teacher2 = new Teacher();
            teacher2.setName("罗老师");
            Student s1 = new Student();
            s1.setName("小强");
            Student s2 = new Student();
            s2.setName("小红");
            s1.getTeachers().add(teacher1);
            s1.getTeachers().add(teacher2);
            // s1 被两个老师交了
            s2.getTeachers().add(teacher1);
            s2.getTeachers().add(teacher2);
            // s2 被两个老师交了
            SqlSession session = MyBatisUtil.getSession();
            TeacherMapper teacherMapper = session.getMapper(TeacherMapper.class);
            StudentMapper studentMapper = session.getMapper(StudentMapper.class);
            teacherMapper.save(teacher1);
            teacherMapper.save(teacher2);
            studentMapper.save(s1);
            studentMapper.save(s2);
            // 往中间表存入数据老师教学生的关系数据
            for (Teacher t : s1.getTeachers()) {
                studentMapper.insertRelation(t.getId(), s1.getId());
            }
            for (Teacher t : s2.getTeachers()) {
                studentMapper.insertRelation(t.getId(), s2.getId());
            }
            session.commit();
            session.close();
        }
    }

2. 单向多对多之额外 SQL 查询

2.1、需求

根据 id 查询学生，并查询其老师。

2.2、需要添加 Mapper 接口 和 Mapper XML

• StudentMapper 接口

package com.yy.mapper;
public interface StudentMapper {
	Student get(Long id);
}

• StudentMapper.xml 文件

<resultMap type="Student" id="baseResultMap">
	<id column="id" property="id"/>
	<result column="name" property="name"/>
	<!-- 关联属性，让 MyBatis 发额外 SQL -->
	<collection select="cn.wolfcode.mapper.TeacherMapper.queryByStudentId" column="id" property="teachers"/>
</resultMap>
<select id="get" resultMap="baseResultMap">
	SELECT id, name FROM student WHERE id = #{id}
</select>

• TeacherMapper XML

package com.yy.mapper;
public interface queryByStudentId {
	Teacher get(Long id);
}

• TeacherMapper.xml 文件

<select id="queryByStudentId" resultType="Teacher">
	SELECT t.id, t.name
	FROM teacher_student ts
	JOIN teacher t ON ts.teacher_id = t.id
	WHERE ts.student_id = #{studentId}
</select>

2.3、单元测试方法

public class Many2oneTest {
	@Test
	public void testGet() throws Exception {
		SqlSession session = MyBatisUtil.getSession();
		StudentMapper studentMapper = session.getMapper(StudentMapper.class);
		Student student = studentMapper.get(2L);
		System.out.println(student); session.close();
	}
}

总结：

以上就是使用 Mybatis 多表实现多对多查询、添加操作了，代码仅供参考，欢迎讨论交流。