code: public <E extends Identifiable> E mergeEntity(EntityManager em, E entity) {
  		requireArgument(em != null, "The entity manager cannot be null.");
  		requireArgument(entity != null, "The entity cannot be null.");
  		E ret = em.merge(entity);
  		return ret;
  	}
docs: Persists an entity to the database.
  
  @param   <E>     The entity type.
  @param   em      The entity manager to use. Cannot be null.
  @param   entity  The entity to persist. Cannot be null.
  
  @return  The persisted entity having all updates applied.