The POJO Class is like this.

public class User
{
private String mId = null;
private final Collection<RescalMem> mRescalMemSet;
}

and hibernate Mapping files is:

<class name="com.User" table="USER" mutable="true" >
<id name = "mId" access="field" column="USR_ID" type="java.lang.String" length="36">
<generator class="assigned"/>
</id>
<set batch-size="250" name="mRescalMemSet" access="field" table="RESCAL_MEM" order-by="INDEXNUM asc" lazy="true" fetch="select" cascade="all-delete-orphan" inverse="true">
<key column="USR_ID"/>
<one-to-many class="com.RescalMem"/>
</set>
</class>

now when the first time i get the data from the database, it's fine.
but when there is something in the data "RescalMem" i want to set the collection as leer, and save the User with a leer RescalMem.

Code like:
session.clear();
User user = session.get(User.class, mUser.getId());
user.getRescalMemSet().clear();
try
{
session.merge(user);
session.commit();
}
catch(Exception e)
{

}

Error occurs:
org.hibernate.HibernateException: collection is not associated with any session
at org.hibernate.collection.AbstractPersistentCollection.forceInitialization(AbstractPersistentCollection.java:471)
at org.hibernate.engine.StatefulPersistenceContext.initializeNonLazyCollections(StatefulPersistenceContext.java:867)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:264)
at org.hibernate.loader.Loader.loadEntity(Loader.java:1881)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:71)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:65)
at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3072)
at org.hibernate.event.def.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:434)
at org.hibernate.event.def.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:415)
at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:165)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:223)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:126)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:905)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:842)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:835)