[#javax-jdo-annotation-PrimaryKey]
= @PrimaryKey (jdo)

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The `@javax.jdo.annotation.PrimaryKey` annotation is used by JDO/DataNucleus to indicate that a property is used as the primary key for an entity with application-managed identity.

Apache Causeway also uses this annotation in a very minimal way: to ensure that the framework's own logic to initialize newly instantiated objects (eg using xref:refguide:applib:index/services/factory/FactoryService.adoc[FactoryService#create(...)] does not touch the primary key, and also to ensure that the primary key property is always disabled (read-only).

[NOTE]
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Apache Causeway parses the `@NotPersistent` annotation from the Java source code; it does not query the JDO metamodel.
This means that it the `@NotPersistent` annotation must be used rather than the equivalent `<field>` link:http://www.datanucleus.org/products/accessplatform_4_0/jdo/application_identity.html[XML metadata].

Moreover, while JDO/DataNucleus will recognize annotations on either the field or the getter method, Apache Causeway (currently) only inspects the getter method.
Therefore ensure that the annotation is placed there.
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