diff --git a/services/od.py b/services/od.py
index 624ad95..73f369a 100644
--- a/services/od.py
+++ b/services/od.py
@@ -261,3 +261,18 @@ def get_attributes(bottom, class_node: UUID):
         # typ = navigate_modelref(bottom, ref_type)
         result.append((attr_name, attr_edge))
     return result
+
+# We need the meta-model (`mm`) to find out how to read the `modelref`
+def read_primitive_value(bottom, modelref: UUID, mm: UUID):
+    typ = get_type(bottom, modelref)
+    if not is_typed_by(bottom, typ, get_scd_mm_modelref_node(bottom)):
+        raise Exception("Assertion failed: argument must be typed by ModelRef")
+    referred_model = UUID(bottom.read_value(modelref))
+    typ_name = get_object_name(bottom, mm, typ)
+    if typ_name == "Integer":
+        return Integer(referred_model, bottom.state).read()
+    elif typ_name == "String":
+        return String(referred_model, bottom.state).read()
+    else:
+        raise Exception("Unimplemented type:", host_type_name)
+
diff --git a/transformation/rewriter.py b/transformation/rewriter.py
index f255b09..d9b9238 100644
--- a/transformation/rewriter.py
+++ b/transformation/rewriter.py
@@ -136,8 +136,8 @@ def rewrite(state, lhs: UUID, rhs: UUID, rhs_mm: UUID, match_mapping: dict, m_to
         model_element_name = match_mapping[pattern_element_name]
         print('updating', model_element_name)
         model_element, = bottom.read_outgoing_elements(m_to_transform, model_element_name)
-        old_value = bottom.read_value(model_element)
-        print('old value:', old_value)
+        # old_value = bottom.read_value(model_element)
+        # print('old value:', old_value)
         host_type = od.get_type(bottom, model_element)
         if od.is_typed_by(bottom, host_type, class_type):
             print(' -> is classs')
@@ -149,29 +149,17 @@ def rewrite(state, lhs: UUID, rhs: UUID, rhs_mm: UUID, match_mapping: dict, m_to
             # referred_model_type = od.get_type(bottom, referred_model_id) # None
             # print('referred_model_type:', referred_model_type)
 
-            host_type_name = od.get_object_name(bottom, mm, host_type)
-            print('host_type_name:', host_type_name)
-            if host_type_name == "Integer":
-                v = Integer(UUID(old_value), state).read()
-            elif host_type_name == "String":
-                v = String(UUID(old_value), state).read()
-            else:
-                raise Exception("Unimplemented type:", host_type_name)
+            v = od.read_primitive_value(bottom, model_element, mm)
 
             # the referred model itself doesn't have a type, so we have to look at the type of the ModelRef element in the RHS-MM:
             rhs_element, = bottom.read_outgoing_elements(rhs, pattern_element_name)
-            rhs_type = od.get_type(bottom, rhs_element)
-            rhs_type_name = od.get_object_name(bottom, rhs_mm, rhs_type)
-            print("rhs_type_name:", rhs_type_name)
 
-            print(od.get_object_name(bottom, mm, model_element))
+            expr = od.read_primitive_value(bottom, rhs_element, rhs_mm)
 
-            if rhs_type_name == "String":
-                python_expr = String(UUID(bottom.read_value(rhs_element)), state).read()
-                result = eval(python_expr, {}, {'v': v})
-                print('eval result=', result)
-                if isinstance(result, int):
-                    # overwrite the old value
-                    Integer(UUID(old_value), state).create(result)
-                else:
-                    raise Exception("Unimplemented type. Value:", result)
+            result = eval(expr, {}, {'v': v})
+            # print('eval result=', result)
+            if isinstance(result, int):
+                # overwrite the old value
+                Integer(referred_model_id, state).create(result)
+            else:
+                raise Exception("Unimplemented type. Value:", result)