Overview of persisting a domain model - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 112

The application does not explicitly call an API method to save an updated
object. Instead, the ORM framework tracks which objects have been changed by
the application and automatically updates the database. Moreover, some ORM
frameworks automatically save or delete an object without the application calling
an API method. They automatically save a nonpersistent object in the database if it
is referenced by another persistent object. An application only needs to save top-
level “root” objects that are not referenced by any other persistent objects. Simi-
larly, some ORM frameworks can be configured to automatically delete a child
object when its parent is deleted or when it is no longer referenced by its parent.
Query language
In addition to loading objects individually, an application often needs to execute
queries that find all matching objects. To do this, it uses the ORM framework's
query language. A query is expressed in terms of objects, their attributes, and rela-
tionships. The query language supports sorting and aggregate functions such as
sum() , min() , and max() . When called by a repository to execute a query, the ORM
framework translates the query into a SQL SELECT statement that retrieves the
objects. Some ORM frameworks also allow the application to retrieve objects using
a SQL query, which is useful when it needs to use database-specific SQL features.
Support for transactions
An application must usually update a database using transactions in order to pre-
serve data integrity. Transactions (which are described in more detail in chapter 12)
ensure, among other things, that if the application fails partway through updating
the database the already made changes will be undone. An ORM framework sup-
ports transactions in a couple of ways. It is integrated with the JTA , which enables
it to be used by applications that update multiple resources, such as a database and
JMS , at the same time. An ORM framework also has an API for managing transactions
directly. The API provides methods for beginning, committing, and rolling back a
transaction. An application can use this transaction management API instead of JTA
if it accesses a single database using the ORM framework.
Lazy and eager loading
As we will see in later chapters, the business tier typically handles a request by first
calling the ORM framework to explicitly load one or more “root” objects and then
navigating to other objects by traversing relationships starting from the root objects.
For instance, the example application loads a PendingOrder and then navigates to
its line item and its restaurant. An important way to improve performance is to opti-
mize the loading of objects by using the right balance of lazy and eager loading.