Session session = ...;

Company company = session.byNaturalId( Company.class )
    .using( "taxIdentifier","abc-123-xyz" )
    .load();

PostalCarrier carrier = session.byNaturalId( PostalCarrier.class )
    .using( "postalCode",new PostalCode(... ) )
    .load();

Department department = ...;
Course course = session.byNaturalId( Course.class )
    .using( "department",department )
    .using( "code","101" )
    .load();