DN v5 : JPA with “nondurable” identity

With standard JPA you only have the ability to have “application” identity for an entity (i.e field(s) marked as being part of the primary-key). Some time ago we added vendor extension support for having a surrogate (datastore) identity for an entity with DataNucleus. DataNucleus v5 now adds a further vendor extension whereby you can have an entity that has no identity. This is useful where you maybe have a class that represents a log entry and you don’t have need to access a specific object (other than searching for all entries with particular field value(s)). You would do this as follows

@Entity
@org.datanucleus.api.jpa.annotations.NonDurableId
public class LogEntry
{
    String level;
    String category;
    String message;

    ...
}

Note that you can specify this in orm.xml also, see the DataNucleus documentation. So we have made use of a DataNucleus-specific annotation, and now we can persist objects of this type and they will not be assigned an identity. We now handle objects of this type in the normal way for querying, just that we cannot use em.find(…).

This is not part of the JPA 2.1 spec, and is not likely to be included in a JPA spec any time soon, but you can make use of it in DataNucleus v5+.