Persisting a domain model with JDO 2.0 - POJOs in Action: Developing Enterprise Applications with Lightweight Frameworks - page 152

primary-key="true" value-strategy="native" />
…
</class>
This example shows part of the PendingOrder class and an excerpt of its XML
metadata. To use application identity with the PendingOrder class we must add
an ID field to store the primary key. It usually makes sense to also define a getter
so that the rest of the application can access the ID . This example configures the
JDO identity of the PendingOrder class using the following attributes:

The identity-type="application" attribute specifies that you want to
use application identity.

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The primary-key="true" attribute specifies that the PendingOrder.id
field will store the primary key.

■

The value-strategy="native" attribute tells the JDO implementation to
pick the most suitable identifier generation strategy based on the underly-
ing database.

■

When the application calls PersistenceManager.makePersistent() to save a
newly created PendingOrder object, the JDO implementation will generate the
primary key using one of a variety of key generation mechanisms, including data-
base sequences and auto-increment columns, and store it in the ID field.
The application can also assign values to the primary key field(s) before calling
PersistenceManager.makePersistent() . This can be useful when you're map-
ping a domain model to a legacy schema that uses a natural primary key instead of
a surrogate primary key. If necessary, either the application can implement its
own key-generation mechanism or it can call JDO to generate a primary key value.
A class that uses application identity must have an object ID class, which
defines fields corresponding to the names of the class's primary key fields. An
application loads an existing object with a particular primary key by passing an
instance of the object ID class that contains the primary key to a method, such as
getObjectById() . If a class has a single primary key field, which is termed single
field identity , then the application uses one of the built-in single field identity pri-
mary key classes. However, if the class has multiple primary key fields, then the
application defines a custom object ID class.
Here is an example of how an application would retrieve a PendingOrder with
an ID of 555 when using application identity:

String idString = "555";
IntIdentity objectId = new IntIdentity(PendingOrder.class, idString);
PendingOrder p = (PendingOrder)pm.getObjectById(objectId);