public IHttpActionResult PutToPayoutPIFromAccount(int accountId, [FromBody] PaymentInstrument paymentInstrument)
        {
            var account = _accounts.Single(a => a.AccountID == accountId);

            account.PayoutPI = paymentInstrument;
            return(Ok(paymentInstrument));
        }
        // POST ~/Accounts(100)/PayinPIs
        public IHttpActionResult PostToPayinPIsFromAccount(int key, PaymentInstrument pi)
        {
            var account = _accounts.Single(a => a.AccountID == key);

            pi.PaymentInstrumentID = account.PayinPIs.Max(p => p.PaymentInstrumentID) + 1;
            account.PayinPIs.Add(pi);
            return(Created(pi));
        }
        public IHttpActionResult PutToPayinPI(int accountId, int paymentInstrumentId, [FromBody] PaymentInstrument paymentInstrument)
        {
            var account    = _accounts.Single(a => a.AccountID == accountId);
            var originalPi = account.PayinPIs.Single(p => p.PaymentInstrumentID == paymentInstrumentId);

            originalPi.FriendlyName = paymentInstrument.FriendlyName;
            return(Ok(paymentInstrument));
        }