fix(nordigen): bring back transaction ID option

Bring back support for the user to change transaction ID. Lets give the
users the power to change stuff while we still attempt to support all
banks with specific mappers out of the box.

Fixes #64

config.go

@@ -77,6 +77,12 @@ type Nordigen struct {
 	// "foo,bar"
 	PayeeStrip []string `envconfig:"NORDIGEN_PAYEE_STRIP"`
 
+	// TransactionID is the field to use as transaction ID. Not all banks use
+	// the same field and some even change the ID over time.
+	//
+	// Valid options are: TransactionId, InternalTransactionId
+	TransactionID string `envconfig:"NORDIGEN_TRANSACTION_ID" default:"TransactionId"`
+
 	// RequisitionHook is a exec hook thats executed at various stages of the
 	// requisition process. The hook is executed with the following arguments:
 	// <status> <link>

reader/nordigen/mapper.go

@@ -21,7 +21,8 @@ func (r Reader) Mapper() Mapper {
 
 	default:
 		return Default{
-			PayeeSource: r.Config.Nordigen.PayeeSource,
+			PayeeSource:   r.Config.Nordigen.PayeeSource,
+			TransactionID: r.Config.Nordigen.TransactionID,
 		}
 	}
 }
@@ -44,7 +45,8 @@ func parseDate(t nordigen.Transaction) (time.Time, error) {
 
 // Default mapping for all banks unless a more specific mapping exists
 type Default struct {
-	PayeeSource []string
+	PayeeSource   []string
+	TransactionID string
 }
 
 // Map t using the default mapper
@@ -91,9 +93,20 @@ func (mapper Default) Map(a ynabber.Account, t nordigen.Transaction) (ynabber.Tr
 		}
 	}
 
+	// Set the transaction ID according to config
+	var id string
+	switch mapper.TransactionID {
+	case "InternalTransactionId":
+		id = t.InternalTransactionId
+	case "TransactionId":
+		id = t.TransactionId
+	default:
+		return ynabber.Transaction{}, fmt.Errorf("unrecognized TransactionID: %s", mapper.TransactionID)
+	}
+
 	return ynabber.Transaction{
 		Account: a,
-		ID:      ynabber.ID(t.TransactionId),
+		ID:      ynabber.ID(id),
 		Date:    date,
 		Payee:   ynabber.Payee(payee),
 		Memo:    t.RemittanceInformationUnstructured,