学习内容：

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 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

INSERT INTO teacher(name) VALUES(#{name})
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

INSERT INTO student(name) VALUES(#{name})
INSERT INTO teacher_student(teacher_id, student_id) VALUES (#{teacherId}, # {studentId})

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();
        }
    }

2.1、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
TeacherMapper XML

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

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();
    }
}