fix: set instrument precision in fromProtoInstrumentAmount adapter

The adapter was creating domain.Instrument without Precision, defaulting
to 0. This caused persistence to reject valid decimal amounts (e.g.,
"100.5" GBP) as "fractional cents". For known ISO 4217 currencies, use
CurrencyToInstrument for correct precision. For unknown instrument codes,
infer precision from the amount string's decimal places.

Also fix two test assertions: proto field test expected "100" but fixture
was "100.00000005", and serialization round-trip expected "999.123456789"
but fixture was "999.123".

Author: bjcoombs

api/proto/meridian/financial_accounting/v1/financial_accounting_test.go

@@ -69,8 +69,8 @@ func TestLedgerPostingCreation(t *testing.T) {
 	if posting.PostingAmount.InstrumentCode != "GBP" {
 		t.Errorf("Expected GBP currency, got %s", posting.PostingAmount.InstrumentCode)
 	}
-	if posting.PostingAmount.Amount != "100" {
-		t.Errorf("Expected 100 amount, got %s", posting.PostingAmount.Amount)
+	if posting.PostingAmount.Amount != "100.00000005" {
+		t.Errorf("Expected 100.00000005 amount, got %s", posting.PostingAmount.Amount)
 	}
 }
 

services/financial-accounting/service/adapters.go

@@ -42,6 +42,10 @@ func parseUUID(s string) (uuid.UUID, error) {
 //
 // The conversion parses the string amount and creates an Instrument from the instrument code.
 // This supports any asset type (currencies, energy, commodities, etc.).
+//
+// For known ISO 4217 currencies (GBP, USD, EUR, etc.), the instrument is created via
+// CurrencyToInstrument which sets the correct dimension and precision. For unknown
+// instrument codes, precision is inferred from the amount string's decimal places.
 func fromProtoInstrumentAmount(ia *quantityv1.InstrumentAmount) (domain.Money, error) {
 	if ia == nil {
 		return domain.Money{}, ErrNilInstrumentAmount
@@ -52,14 +56,38 @@ func fromProtoInstrumentAmount(ia *quantityv1.InstrumentAmount) (domain.Money, e
 		return domain.Money{}, fmt.Errorf("invalid amount: %w", err)
 	}
 
+	// Try known currency codes first for backward compatibility.
+	// This preserves the correct dimension ("CURRENCY") and precision (e.g., 2 for GBP).
+	if currency, currErr := domain.ParseCurrency(ia.InstrumentCode); currErr == nil {
+		instrument, instErr := domain.CurrencyToInstrument(currency)
+		if instErr == nil {
+			return domain.NewMoney(amount, instrument), nil
+		}
+	}
+
+	// For non-currency instrument codes, infer precision from the amount string.
+	precision := inferPrecisionFromAmount(ia.Amount)
+
 	instrument := domain.Instrument{
-		Code:    ia.InstrumentCode,
-		Version: uint32(ia.Version),
+		Code:      ia.InstrumentCode,
+		Version:   uint32(ia.Version),
+		Precision: precision,
 	}
 
 	return domain.NewMoney(amount, instrument), nil
 }
 
+// inferPrecisionFromAmount derives the number of decimal places from an amount string.
+// For example: "100.50" returns 2, "1.234567" returns 6, "100" returns 0.
+func inferPrecisionFromAmount(amount string) int {
+	for i := len(amount) - 1; i >= 0; i-- {
+		if amount[i] == '.' {
+			return len(amount) - 1 - i
+		}
+	}
+	return 0
+}
+
 // toProtoInstrumentAmount converts domain Money to protobuf InstrumentAmount.
 // Preserves full decimal precision via string representation.
 func toProtoInstrumentAmount(m domain.Money) *quantityv1.InstrumentAmount {

services/financial-accounting/service/financial_accounting_service_test.go

@@ -1440,7 +1440,7 @@ func TestCaptureLedgerPosting_IdempotencySerializationRoundTrip(t *testing.T) {
 		assert.Equal(t, bookingLogID.String(), deserializedResponse.LedgerPosting.FinancialBookingLogId)
 		assert.Equal(t, commonv1.PostingDirection_POSTING_DIRECTION_CREDIT, deserializedResponse.LedgerPosting.PostingDirection)
 		assert.Equal(t, "EUR", deserializedResponse.LedgerPosting.PostingAmount.InstrumentCode)
-		assert.Equal(t, "999.123456789", deserializedResponse.LedgerPosting.PostingAmount.Amount)
+		assert.Equal(t, "999.123", deserializedResponse.LedgerPosting.PostingAmount.Amount)
 		assert.Equal(t, "ACC-456", deserializedResponse.LedgerPosting.AccountId)
 		assert.Equal(t, commonv1.TransactionStatus_TRANSACTION_STATUS_PENDING, deserializedResponse.LedgerPosting.Status)
 		assert.Equal(t, "test-result", deserializedResponse.LedgerPosting.PostingResult)