JPA and MySQL mapping entites

Question

@Entity
@Table(name = "COURSE")
public class Course {

    @Id
    @GeneratedValue
    private Long id;

    @Column(name = "course_name")
    private String courseName;

    @ManyToOne
    Department department;

    @ManyToOne
    Student student;

    protected Course() {}

    public Course(String name, Department department) {
        this.department = department;
        courseName = name;
    }

}



@Entity
@Table(name = "STUDENT")
public class Student {
    @Id
    @GeneratedValue
    private Long id;

    @Column(name = "locker_id")
    private int lockerId;

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "student",
            cascade = CascadeType.ALL)
    List<Course> courses = new ArrayList<>();

    @Embedded
    private Person attendee;

    protected Student(){}

    public Student(Person person, int lockerId) {
        attendee = person;
        this.lockerId = lockerId;
        courses = new ArrayList<>();
    }

    public void setCourse(Course course) {
        courses.add(course);
    }

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

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

}



@SpringBootApplication
public class UniversityApplication implements CommandLineRunner {

    @Autowired
    CourseRepository courseRepository;
    @Autowired
    DepartmentRepository departmentRepository;
    @Autowired
    StudentRepository studentRepository;

    public static void main(String[] args) {
        SpringApplication.run(UniversityApplication.class, args);
    }

    @Override
    public void run(String... args) throws Exception {

        //Students
        Student one = studentRepository.save(new Student(new Person("jane", "doe"), 20));

        //Courses
        Course english101 = courseRepository.save(new Course("English 101", humanities));
        Course english202 = courseRepository.save(new Course("English 202", humanities));

        //This does not add student to a course, why?
        //Ask
        one.setCourse(english101);
        studentRepository.save(one);
        //How to map course with student and then to find students in a particular course

    }
}

I have already successfully mapped Department and Course, in course you can find the department id. I want the same thing to work for Student class, so that I can find Students id in MySQL table @ Course.

I want to add student to a specific course and save it and that does not seem to work either.

Okey, I do not get any errors actually. But in the MySQL database i cant see the students ids? — oneoe, Apr 23 '18 at 11:48
Try removing `studentRepository.save()` from the `Student one` initialisation. So just `new Student(....)` — , Apr 23 '18 at 11:51
I have tried that also, it seems like department is working correct because it is in the course constructor? — oneoe, Apr 23 '18 at 11:53
I feel the issue might lay in you saving an Object during an assignment. Have you tried to separate all of them? (so also the courseRepository saves). Also, be consistent in the use of `this` in your classes — , Apr 23 '18 at 11:55

score 1 · Answer 1 · answered Apr 23 '18 at 12:24

1

The problem is that your @ManyToOne relationship doesn't know how to connect the tables. Please change:

 @ManyToOne
Student student;

to:

@ManyToOne
@JoinColumn(name = "student_id")
Student student;

here is a good explanation about @JoinColumns and "mappedBy"

answered Apr 23 '18 at 12:24

Kai

61
2

Thank you for the answer and for the extra explanation link. – oneoe Apr 23 '18 at 13:38
If this answer works for you, please accept it by hitting the green tick – Al-un Apr 24 '18 at 03:57
Actually it did not work, I thought this was it but it did not. – oneoe Apr 24 '18 at 07:17

score 0 · Answer 2 · answered Apr 23 '18 at 12:20

0

Try to enable query generation log, to check what queries exactly are generated. If you will not see any INSERT/UPDATE queries I would assume problems with transactions. Need to make you call transactional.

answered Apr 23 '18 at 12:20

Alex

98
1
8

score 0 · Accepted Answer · answered May 28 '18 at 09:53

0

   public void setCourse(Course course) {
        courses.add(course);
        course.setStudent(this);
    }

I just had to set the Student for this course in this method to make the mapping work.

answered May 28 '18 at 09:53

oneoe

53
1
2
8

JPA and MySQL mapping entites

3 Answers3