Overview of persisting a domain model - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 146

consider improving the performance of the query that finds the available restau-
rants. One option is to use the query cache. Enabling caching for this query
might improve performance because JDO or Hibernate will then try to get the
results of the query from the query cache instead of executing a SQL statement.
However, because there are potentially many combinations of values for the
query's parameters—for example, ZIP code and delivery time—it is unclear
whether there would be any benefit. We would have to analyze the running appli-
cation to determine whether there is any benefit. The query cache can improve
performance of other queries significantly.
By using eager loading, process-level caching, and query caching, you can
often significantly improve the performance of your application. The details
depend on which persistence framework your application uses. You will learn
about those in the next two chapters, but before that we must look at the details of
the example schema.
4.7 The example schema
We have now had an overview of ORM and looked at how to write tests and opti-
mize the performance of a persistence layer. The next step is to learn how to per-
sist a domain model using Hibernate and JDO . Because I'm going to use the Food
to Go domain model as an example, it will be helpful to look at the database
schema that the application uses. The schema, which is shown in figure 4.5, is
quite similar to the domain model you saw in chapter 3.
It uses many of the ORM techniques you saw earlier in section 4.1. Each of the
main classes in the domain model— PendingOrder , PendingOrderLineItem , Order ,
OrderLineItem , Coupon , Restaurant , and MenuItem —has a corresponding table.
The Restaurant - MenuItem and PendingOrder - PendingOrderLineItem relationships,
which are one-to-many relationships, are represented by foreign keys in the
MENU_ITEM and PENDING_ORDER_LINE_ITEM tables, respectively.
Similarly, the PendingOrder - PendingOrderLineItem , which is a many-to-one rela-
tionship, is represented by a foreign key in the PENDING_ORDER_LINE_ITEM table.
A restaurant's ZIP codes—which in Java is a collection of strings and corresponds
to a many-to-many relationship between restaurants and ZIP codes—is represented
by the join table RESTAURANT_ZIP_CODE , which has foreign keys to both the RES-
TAURANT and ZIP_CODE tables.
In the following chapters, this schema is used to illustrate some of the chal-
lenges you will face when trying to Hibernate or JDO to persist a domain model.