I am using a List field in my domain model class. It represents many-to many association. It is umidirectional association betwen Class1 and Class2. In mapping I am using <bag> for this list.

The QUESTION is: Why hibernate recreates list (mapped as <bag> reprezenting many-to-many association) each time I simply add new element to it?

Hibernate version:

2.1.6

Domain model classes:

package jbeans;

public class Class2 {

private Long id;

public Long getId() {
return id;
}

public void setId(Long id) {
this.id = id;
}

}

package jbeans;

import java.util.*;

public class Class1 {

private Long id;

private List classes2;

public Long getId() {
return id;
}

public void setId(Long id) {
this.id = id;
}

public List getClasses2() {
return classes2;
}

public void setClasses2(List classes2) {
this.classes2 = classes2;
}

public void addClass2(Class2 class2){
classes2.add(class2);
}
}

Mapping documents:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping package="jbeans">
<class
name="Class1"
table="CLASSES1">
<id
name="id"
type="long"
column="ID"
>
<generator class="native"/>
</id>
<bag
name="classes2"
lazy="true"
table="CLASSES1_CLASESS2"
>
<key column="CLASS1_ID"/>
<many-to-many class="Class2" column="CLASS2_ID"/>
</bag>
</class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping package="jbeans">
<class
name="Class2"
table="CLASSES2">
<id
name="id"
type="long"
column="ID"
>
<generator class="native"/>
</id>
</class>
</hibernate-mapping>

Code between sessionFactory.openSession() and session.close():

/*Insert new Class1*/
/*This is executed only once after database creation*/
tr = ses.beginTransaction();
Class1 c11 = new Class1();
ses.save(c11);
tr.commit();
ses.close();

/*Insert new Class2 and associate with Class1*/
/*This is executed few times after new Class1 insert in new Hibernate session*/
tr = ses.beginTransaction();
Criteria criteria = ses.createCriteria(Class1.class);
List result = criteria.list();
Class1 c12 = (Class1)result.get(0);
Class2 c21 = new Class2();
ses.save(c21);
c12.getClasses2().add(c21);
tr.commit();

Name and version of the database I am using:
MySQL 4.0.13

The generated SQL (show_sql=true):

/*When Insert new Class1 is executed*/
Hibernate: insert into CLASSES1 values ( )

/*When Insert new Class2 and associate with Class1 is execited*/

/*First time*/
Hibernate: select this.ID as ID0_ from CLASSES1 this where 1=1
Hibernate: insert into CLASSES2 values ( )
Hibernate: select classes20_.CLASS1_ID as CLASS1_ID__, classes20_.CLASS2_ID as CLASS2_ID__, class21_.ID as ID0_ from CLASSES1_CLASESS2 classes20_ inner join CLASSES2 class21_ on classes20_.CLASS2_ID=class21_.ID where classes20_.CLASS1_ID=?
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)


/*Second time*/
Hibernate: select this.ID as ID0_ from CLASSES1 this where 1=1
Hibernate: insert into CLASSES2 values ( )
Hibernate: select classes20_.CLASS1_ID as CLASS1_ID__, classes20_.CLASS2_ID as CLASS2_ID__, class21_.ID as ID0_ from CLASSES1_CLASESS2 classes20_ inner join CLASSES2 class21_ on classes20_.CLASS2_ID=class21_.ID where classes20_.CLASS1_ID=?
Hibernate: delete from CLASSES1_CLASESS2 where CLASS1_ID=?
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)

/*Third time*/
Hibernate: select this.ID as ID0_ from CLASSES1 this where 1=1
Hibernate: insert into CLASSES2 values ( )
Hibernate: select classes20_.CLASS1_ID as CLASS1_ID__, classes20_.CLASS2_ID as CLASS2_ID__, class21_.ID as ID0_ from CLASSES1_CLASESS2 classes20_ inner join CLASSES2 class21_ on classes20_.CLASS2_ID=class21_.ID where classes20_.CLASS1_ID=?
Hibernate: delete from CLASSES1_CLASESS2 where CLASS1_ID=?
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)
Hibernate: insert into CLASSES1_CLASESS2 (CLASS1_ID, CLASS2_ID) values (?, ?)

Etc.

Please read the Hibernate reference documentations. (The section on collection performance.)

OK.
So I tried to use <idbag> instead of <bag>. It works much better. One problem is that when I want to insert new element a full list is loaded from database. Besides it creates surrogate key... I have read that a solution of this problem is mapping this association as two bidirectional one-to-many associations with additional association class and declare this collections as (in my case inverse <bag>). Is it the only sollution of this problem?