Persisting a domain model with Hibernate 3 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 205

cascade="none" />
</composite-element>

</list>
…
</class>
This mapping specifies that the lineItems property is a collection of Pending-
OrderLineItem components that is stored in the PENDING_ORDER_LINE_ITEM
table. Notice that because PendingOrderLineItem is mapped as a component it
does not have a separate <class> mapping.
Here is the definition of the PENDING_ORDER_LINE_ITEM table:

CREATE TABLE PENDING_ORDER_LINE_ITEM (
QUANTITY NUMBER(10) NOT NULL,
PENDING_ORDER_ID NUMBER(10) NOT NULL,
LINE_ITEM_INDEX NUMBER(10) NOT NULL,
MENU_ITEM_ID NUMBER(10),
)
The PENDING_ORDER_ID column is a foreign key to the PENDING_ORDER table, and
the MENU__ITEM_ID column is a foreign key to the MENU_ITEM table. Notice that
this table does not have a surrogate primary key. Instead, the primary key consists
of the PENDING_ORDER_LINE_ITEM_ID and LINE_ITEM_INDEX columns.
A benefit of using component collections is that Hibernate persists a newly cre-
ated child by executing a single INSERT SQL statement. There are, however, a cou-
ple of limitations. The child table cannot use a surrogate key, which means, for
example, that Hibernate will not set a surrogate primary key column to a gener-
ated value. If the table must have a surrogate key—in order to comply with data-
base schema design guidelines, for instance—the application must use a trigger to
initialize the primary key.
Another important limitation of component collections is that the child class
can only be referenced by its parent. Because of this restriction, we cannot use a
component collection for the Restaurant - MenuItem relationship since menu items
are also referenced by pending order line items and order line items. We can,
however, use component collections for the PendingOrder - PendingOrderLineItem
and Order - OrderLineItem relationships because line items are referenced only by
their parent.

6.1.4 Using the cascade attribute

In chapter 4 we saw that an ORM framework must participate in the creation and
destruction of a persistent object in order to update the database. For example,