[payments] add method to Invoice for getting at the user and username

extra/modules/payments.py

@@ -27,6 +27,7 @@ from qommon.form import htmltext, StringWidget, TextWidget, SingleSelectWidget
 from wcs.formdef import FormDef
 from wcs.formdata import Evolution
 from wcs.workflows import WorkflowStatusItem, register_item_class, template_on_formdata
+from wcs.users import User
 
 def is_payment_supported():
     if not eopayment:
@@ -77,6 +78,16 @@ class Invoice(StorableObject):
         if get_publisher() and not self.id:
             self.id = self.get_new_id()
 
+    def get_user(self):
+        if self.user_id:
+            return User.get(self.user_id, ignore_errors=True)
+        return None
+
+    @property
+    def username(self):
+        user = self.get_user()
+        return user.name if user else ''
+
     def get_new_id(self, create=False):
         # format : date-regie-formdef-alea-check
         r = random.SystemRandom()