diff --git a/db/intf.go b/db/intf.go
index cf2fdca..d2f77ee 100644
--- a/db/intf.go
+++ b/db/intf.go
@@ -10,3 +10,7 @@ type UserIdentifiable interface {
 	GetUserID() uint
 	SetUserID(userID uint)
 }
+
+type PaymentGroup interface {
+	__internalBelogingToPayment()
+}