@OneToMany(cascade = Array(CascadeType.ALL))
@OrderBy
private var _items: List[InvoiceItem] = new ArrayList

@Basic(optional = false)
private var _totalAmount: BigDecimal = BigDecimal.ZERO

@Temporal(TemporalType.DATE)
private var _sentDate: Date = _

def sent_? = _sentDate != null

def removeItem(index: Int) {
  require(!sent_?, "items cannot be changed after invoice is sent")
  val item = _items.remove(index)
  _totalAmount = _totalAmount.subtract(item.amount)
}