JPA Objects does not support maps, only lists. This is because Naked Objects itself does not support maps. The JPA @MapKey annotation may therefore not be used.

A workaround is for the entity to provide an action that runs a repository query to locate the required object. One of the properties of the associated class would act as its key, but this fact would not be exposed in the domain model, only in the database schema. Alternatively, your domain object can build a transient map from a persisted collection.