<class name="Order" table="ORDERS">
<id name="id" column="ID"/>
<set name="items">
<key column="id"/>
<one-to-many class="Item"/>
</set>
</class>
<class name="Item" table="ITEMS">
<id name="id" column="ID"/>
<property name="info"/>
</class>

======================
Order order = (Order) session.get(Order.class, new Long(1));
Iterator it = order.getItems().iterator();
while (it.hasNext())
{
Item item = (Item) it.next();
item.setInfo("hello world");
}
tx.commit();
======================
Exception occur [constriant violation of Item table primary key]. Instead of updating an existing record, the log file indicates that hibernate tried to insert a record with an already exist primary key. Why??

Your mapping does not make a lot of sense to me. I would expect something like

This defines that table ITEM contains a foreign key to ORDERS.

Was this a cut & paste error or do you mean it? If the latter, could you please provide your database create statements or more information on what you intend to do.

Erik