org.hibernate.NonUniqueObjectException

[indu iyengar]

Hi all, when I started to use relationships, it ends up with different types of exceptions.. please give tell me what is key element and why index as the element in list ?

am using 3 tables..
student (studID primary key),courses(courseID primary key) and student-course(studID and courseID as columns )..

the student can select different courses.

so one student with


mapping file..

<hibernate-mapping package="com.bean">
  <class name="RegisterStudent" table="details_student">
   <id name="stid" type="long" column="studID">
       <generator class="increment"/>
   </id>
   
        <property name="stname" column="studName"></property>
        <property name="stpw" column="studPW"></property>
        <property name="dob" column="DOB"></property>
                
	  <list name="listOfCourses" table="stud_course" cascade="all">
	     <key column="studID"></key>
	     <index column="idx"></index>
	     <many-to-many column="courseID" class="Course" unique="true"></many-to-many>
	  </list>
	   
	</class>   
	   <class name="Course" table="courses">
	      <id name="courseID" type="long">
	             </id>
	       <property name="courseName" column="courseName"></property>
	       <property name="creditPoints" column="creditPoints"></property>  
   
  </class>
 </hibernate-mapping>

when I call session.save(obj).. then

org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session