Persisting a domain model with Hibernate 3 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 225

Defining the O/R mapping
Now that we have made that simple change to PendingOrder , the other thing we
must do is to write the O/R mapping document, which describes how its fields and
relationships map to the database schema. Let's first examine how the Pending-
Order class, its fields, and its relationships are mapped to the database schema;
after that we'll look at the Hibernate mapping document.
In chapter 4 we described how the PendingOrder class is mapped to the
PENDING_ORDER table. The id field is mapped to the PENDING_ORDER_ID column,
which is the table's primary key. Its simple fields are mapped to columns of this
table. The state field is mapped to the STATE column, and the deliveryTime field
is mapped to the DELIVERY_TIME column.
The deliveryAddress and paymentInformation fields reference embedded
value objects, and so the fields of the Address and PaymentInformation objects are
mapped to the columns of the PENDING_ORDER table. For example, the street1
field of the Address object is mapped to the DELIVERY_STREET1 column of the
PENDING_ORDER table.
The PendingOrder - PendingOrderLineItem relationship is an ordered, unidirec-
tional, one-to-many relationship. In addition, a PendingOrderLineItem must be
deleted when its PendingOrder is deleted or when it is no longer associated with a
PendingOrder . We saw in section 6.1.1 that Hibernate provides two ways to map this
kind of relationship: as an entity collection or as a component collection. Because
the line items are only referenced by PendingOrder , we can use a component col-
lection, which is slightly more efficient because Hibernate uses fewer SQL state-
ments. PendingOrderLineItems are mapped to the PENDING_ORDER_LINE_ITEM
table, which has a foreign key to the PENDING_ORDER table, and a LINE_ITEM_INDEX
column that stores the position of the line item.
The PendingOrder - Restaurant and PendingOrder - Coupon relationships are uni-
directional, many-to-one relationships. The PENDING_ORDER table has a
RESTAURANT_ID column, which is a foreign key to the RESTAURANT table, and a
COUPON_ID column, which is a foreign key to the COUPON table.
Listing 6.3 shows PendingOrder.hbm.xml, which is the O/R mapping document
for PendingOrder . PendingOrder.hbm.xml must be accessible at runtime and is
often located in the same directory of the class and on the class path. In chapter 7,
we will look at how to create a SessionFactory that uses PendingOrder.hbm.xml.