diff --git a/commerce/order.py b/commerce/order.py
index e10e58f..98ea779 100644
--- a/commerce/order.py
+++ b/commerce/order.py
@@ -78,7 +78,7 @@ class OrderItem(Track):
         self.getParent().context.removeTrack(self)
 
     def modify(self, quantity, **kw):
-        self.data['quantity'] = quantity
+        self.update(dict(quantity=quantity))
         return self
 
     def setOrder(self, order):
diff --git a/tracking/btree.py b/tracking/btree.py
index dff2479..86d9534 100644
--- a/tracking/btree.py
+++ b/tracking/btree.py
@@ -86,7 +86,7 @@ class Track(Persistent):
         self.timeStamp = getTimeStamp()
         data = self.data
         data.update(newData)
-        self.data = data
+        self.data = data    # record change
 
     def __repr__(self):
         md = self.metadata