一对多

多对多关系：

学生有多门课程

课程也有许多学生去学

package com.hc.model;

import java.util.List;

public class Student {

    private int stuId;
    private String stuName;
    private List<Course> courses;

    public Student() {
    }

    public Student(String stuName) {
        this.stuName = stuName;
    }

    public int getStuId() {
        return stuId;
    }

    public void setStuId(int stuId) {
        this.stuId = stuId;
    }

    public String getStuName() {
        return stuName;
    }

    public void setStuName(String stuName) {
        this.stuName = stuName;
    }

    public List<Course> getCourses() {
        return courses;
    }

    public void setCourses(List<Course> courses) {
        this.courses = courses;
    }
}

学生类，有一个课程的list属性

package com.hc.model;

import java.util.List;

public class Course {

    private int couId;
    private String couName;
    private List<Student> students;

    public Course() {
    }

    public Course(String couName) {
        this.couName = couName;
    }

    public int getCouId() {
        return couId;
    }

    public void setCouId(int couId) {
        this.couId = couId;
    }

    public String getCouName() {
        return couName;
    }

    public void setCouName(String couName) {
        this.couName = couName;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

课程类，有一个学生的list

package com.hc.mapper;

import com.hc.model.Student;
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")
    @Results({
            @Result(id = true,property = "stuId",column = "stuId"),
            @Result(property = "courses",column = "stuId",many = @Many(select = "com.hc.mapper.CourseMapper.selectCourseBy"))
    })
    public List<Student> allStudent();

    @Select("select * from student where stuId in(select stuId from stuCou where couId=#{couId})")
    public List<Student> selectStudentBy(int couId);

}

学生Mapper

如果，id方面分不清，重要的id或可关联的属性，最好先单独拿出来

package com.hc.mapper;

import com.hc.model.Course;
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 CourseMapper {

    @Select("select * from Course where couId in(select couId from stuCou where stuId=#{stuId})")
    public List<Course> selectCourseBy(int stuId);

    @Select("select * from Course")
    @Results({
            @Result(id = true,property = "couId",column = "couId"),
            @Result(property = "students",column = "couId",many = @Many(select = "com.hc.mapper.StudentMapper.selectStudentBy"))
    })
    public List<Course> allCourse();

}

课程Mapper

多对多的关系，是通过一个第三方表维护的，这里使用的是子查询。

先从第三方表匹配学生的id，通过这个学生id拿到课程id，接着，相对应的课程对象也就能被拿到

package com.hc.test;

import com.hc.mapper.CourseMapper;
import com.hc.mapper.StudentMapper;
import com.hc.model.Course;
import com.hc.model.Student;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

public class StudentTest {

    private SqlSession session;

    @Before
    public void before() {
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(getClass().getClassLoader().getResourceAsStream("myBatis-config.xml"));
        session = factory.openSession();
    }

    @Test
    public void testCourse() {
        CourseMapper courseMapper = session.getMapper(CourseMapper.class);
        for (Course course : courseMapper.allCourse()) {
            System.out.println(course.getCouId() + "\t" + course.getCouName());
            for (Student student : course.getStudents()) {
                System.out.println(student.getStuId() + "\t" + student.getStuName());
            }
            System.out.println();
        }
    }

    @Test
    public void testStudent() {

        StudentMapper studentMapper = session.getMapper(StudentMapper.class);
        for (Student student : studentMapper.allStudent()) {
            System.out.println(student.getStuId() + "\t" + student.getStuName());
            for (Course course : student.getCourses()) {
                System.out.println(course.getCouId() + "\t" + course.getCouName());
            }
            System.out.println();
        }
    }

    @After
    public void after() {
        session.commit();
        session.close();
    }

}

测试

一对一、一对多、多对多，三种关系其实有很多相同之处。

理解其一，就能互通。

不得不说，框架是真的强大！