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

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    <p>I have a ManyToMany association between two Entities: COURSE and STUDENT. They are associated through a STUDENT_COURSE_MAP association table.</p> <p><strong>Course.java:</strong></p> <pre><code>@Entity @Table(name="COURSE") public class Course implements java.io.Serializable { @ManyToMany( mappedBy="courses", cascade={CascadeType.PERSIST, CascadeType.MERGE} ) private List&lt;Student&gt; students = Collections.emptyList(); // Rest of the class, getters, setters etc } </code></pre> <p><strong>Student.java:</strong></p> <pre><code>@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&lt;Course&gt; courses = Collections.emptySet(); // Rest of the class, getters, setters etc } </code></pre> <p>I want to add a student (id=11) list for one particular course (id=77) as follows (transaction is already begun):</p> <pre><code>// Create new list and add a student to it Student student_11 = (Student) session.get(Student.class.getName(), 11); List&lt;Student&gt; studentsList = new ArrayList&lt;Student&gt;(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); </code></pre> <p>I save it using the following code:</p> <pre><code>session.flush(); session.getTransaction().commit(); </code></pre> <p>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.</p> <p>I even tried calling <code>session.saveOrUpdate(course_77)</code>. Only the following is logged when <code>saveOrUpdate</code> is called:</p> <pre><code>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] </code></pre> <p>Hints/tips/solutions are much appreciated.</p> <hr> <p><strong>Note 1:</strong> 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.</p>
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