CODEWITHSUNDEEP
hibernate-mapping
pballs
pballs

Reputation: 313

Hibernate: Adding a new element to association list does not persist

I have a ManyToMany association between two Entities: COURSE and STUDENT. They are associated through a STUDENT_COURSE_MAP association table.

Course.java:

@Entity
@Table(name="COURSE")
public class Course implements java.io.Serializable {
   @ManyToMany(
      mappedBy="courses",
      cascade={CascadeType.PERSIST, CascadeType.MERGE}
   )
   private List<Student> students = Collections.emptyList();

   // Rest of the class, getters, setters etc
}

Student.java:

@Entity
@Table(name="STUDENT")
public class Student implements java.io.Serializable {
   @ManyToMany(
      targetEntity=Course.class,
      cascade = { CascadeType.PERSIST, CascadeType.MERGE }
   )
   @JoinTable(
      name="STUDENT_COURSE_MAP",
      joinColumns=@JoinColumn(name="STUDENT_REF"),
      inverseJoinColumns=@JoinColumn(name="COURSE_REF")
   )
   private Set<Course> courses = Collections.emptySet();

   // Rest of the class, getters, setters etc
}

I want to add a student (id=11) list for one particular course (id=77) as follows (transaction is already begun):

// Create new list and add a student to it
Student student_11 = (Student) session.get(Student.class.getName(), 11);
List<Student> studentsList = new ArrayList<Student>(1);
studentsList.add(student_11);

// Now, set the list of students in the course
Course course_77 = (Course) session.get(Course.class.getName(), 77);
course_77.setStudents(studentsList);

I save it using the following code:

session.flush();
session.getTransaction().commit();

However, the association - course_77 and student_11 - does not get persisted to the database. There are no insert statements generated in the hibernate log.

I even tried calling session.saveOrUpdate(course_77). Only the following is logged when saveOrUpdate is called:

allowing proxied method [saveOrUpdate] to proceed to real session
persistent instance of: org.whatra.tradecog.db.entity.Artist
ignoring persistent instance
object already associated with session: [Course#77]

Hints/tips/solutions are much appreciated.

Note 1: The whole persistence thing works just fine for me when I create new Course and Student objects and save. What does not work is when I retrieve existing Course and existing Student objects and add a new association between them.

Upvotes: 5

Views: 10753

Answers (1)

pballs
pballs

Reputation: 313

Here's what was wrong. It turns out I was saving only one side of the association. This is what I should have done:

// Create new list and add a student to it
Student student_11 = (Student)session.get(Student.class.getName(), 11);
Course course_77 = (Course) session.get(Course.class.getName(), 77);

List studentsList = new ArrayList(1);
studentsList.add(student_11);

Set coursesList = new HashSet(1);
coursesList.add(course_77);

course_77.setStudents(studentsList);
// THIS IS WHAT'S WRONG - I MISSED THIS NEXT LINE
student_11.setCourses(coursesList);

I found the answer at http://en.wikibooks.org/wiki/Java_Persistence/Relationships (look under 'Common Problems').

Upvotes: 6

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