I did this a few days ago, I had an intermediate table like this:
-------------------------
|COURSE_ID | STUDENT_ID |
------------------------
|0 |1 |
|0 |2 |
|1 |1 |
|1 |2 |
-------------------------
but then I wanted to add fields to it (and didn't commit it to github ¬¬, I was in a hurry) and ended up breaking the whole thing up... Long story short I started this example again. My code now creates the two tables "courses" and "students" but they don't reflect anything about the relationship between them, I mean not intermediate table is created; though I can ask for the courses to every student and vice versa in the object world, in the databases world none of this is reflected. Here is my code:
Student class package model;
import java.util.HashSet;
import javax.persistence.JoinColumn;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
import java.util.Set;
@Entity
@Table(name="STUDENTS")
public class Student
{
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id ;
private String name ;
@ManyToMany(targetEntity=Course.class, mappedBy="courseName", fetch=FetchType.EAGER)
private Set<Course> courses ;
public Student(String myName)
{
this.name = myName ;
courses = new HashSet<Course>() ;
}
public void addCourse(Course myCourse)
{
this.courses.add(myCourse) ;
}
@Column(name="STUDENT_ID")
public int getStudent_id()
{
return this.id ;
}
@JoinTable(name="COURSE_STUDENTS",
joinColumns=
@JoinColumn(name="STUDENT_ID"),
inverseJoinColumns=
@JoinColumn(name="COURSE_ID")
)
public Set<Course> getCourses()
{
return this.courses ;
}
@Column(name="STUDENT_NAME")
public String getStudent_name()
{
return this.name ;
}
}
Course class
package model;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;
@Entity
@Table(name="COURSES")
public class Course
{
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private int id ;
@Column(name="COURSE_NAME")
private String courseName ;
@ManyToMany(targetEntity=Student.class, mappedBy="name", fetch=FetchType.EAGER)
private Set<Student> students ;
public Course(String myName)
{
this.courseName = myName ;
this.students = new HashSet<Student> () ;
}
public void addStudent(Student myStudent)
{
this.students.add(myStudent) ;
}
@ManyToMany(mappedBy="courses")
public Set<Student> getCourse_students()
{
return this.students ;
}
@Column(name="COURSE_ID")
public int getCourse_id()
{
return id ;
}
}
Main class that tests this
package controller;
import javax.persistence.EntityManager;
import db.EntityManagerHelper;
import model.Student;
import model.Course;
public class Main {
public static void main(String[] args) {
EntityManager entityManager= EntityManagerHelper.getEntityManager();
Course course_kK = new Course("K1000") ;
Course course_mK = new Course("M1000") ;
Course course_rK = new Course("R1000") ;
Student student_rick = new Student("Rick") ;
student_rick.addCourse(course_rK);
student_rick.addCourse(course_mK);
Student student_rock = new Student("Rock") ;
student_rock.addCourse(course_rK);
student_rock.addCourse(course_kK);
Student student_jack = new Student("Jack") ;
student_jack.addCourse(course_mK);
student_jack.addCourse(course_kK);
course_kK.addStudent(student_rock);
course_kK.addStudent(student_jack);
course_mK.addStudent(student_jack);
course_mK.addStudent(student_rick);
course_rK.addStudent(student_rock);
course_rK.addStudent(student_rick);
System.out.println("course_rK:id = "+course_rK.getCourse_id());
System.out.println("course_kK:id = "+course_kK.getCourse_id());
System.out.println("course_mK:id = "+course_mK.getCourse_id());
EntityManagerHelper.beginTransaction();
entityManager.persist(course_rK);
entityManager.persist(course_kK);
entityManager.persist(course_mK);
entityManager.persist(student_rick);
entityManager.persist(student_rock);
entityManager.persist(student_jack);
entityManager.flush();
EntityManagerHelper.commit();
System.out.println("student_jack:id = "+student_jack.getStudent_id());
System.out.println("student_rock:id = "+student_rock.getStudent_id());
System.out.println("student_gast:id = "+student_rick.getStudent_id());
}
}
How can I create that intermediate table between those two entities that "brakes" the many-to-many relationship?
Here's the detailed answer. I had to make changes in a few lines... I checked out this tutorial (again), the same I first followed when I was able to do what I asked in my original question.
where it said
@Column(name="COURSE_ID")
I changed that to
@Column(name="COURSE_ID", unique=true, nullable=false)
where it said
@Column(name="STUDENT_ID")
I changed that to
@Column(name="STUDENT_ID", unique=true, nullable=false)
where it said
@ManyToMany(targetEntity=Course.class, mappedBy="courseName", fetch=FetchType.EAGER)
I changed that to
@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
where it said
@ManyToMany(mappedBy="courses")
I changed that to
@ManyToMany(fetch = FetchType.LAZY, mappedBy = "courses")
And where it said
@JoinTable(name="COURSE_STUDENTS",
joinColumns=
@JoinColumn(name="STUDENT_ID"),
inverseJoinColumns=
@JoinColumn(name="COURSE_ID")
)
public Set<Course> getCourses()
{
return this.courses ;
}
I changed that to
@JoinTable(name = "STUDENT_COURSE", joinColumns = {
@JoinColumn(name = "STUDENT_ID", nullable = false, updatable = false) },
inverseJoinColumns = { @JoinColumn(name = "COURSE_ID",
nullable = false, updatable = false) }
)
public Set<Course> getCourses()
{
return this.courses ;
}
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