Как программно внедрить контекст сохранения в другой источник данных

In standard EJB 3, when injecting entity manager, persistence unit (which refers to datasource) is hardcoded into annotation: (or alternatively xml file)

@PersistenceContext(unitName = "myunit")
private EntityManager entityManager;

Is there a way to use an entity manager but to select data source by name at runtime?