fix: add serialize_object() method for ObjectRef schema node

so we can use this node type for non-wutta mapped class, with non-uuid
primary key

src/wuttaweb/forms/schema.py

@@ -334,8 +334,21 @@ class ObjectRef(colander.SchemaType):
         # nb. keep a ref to this for later use
         node.model_instance = appstruct
 
-        # serialize to uuid
-        return appstruct.uuid.hex
+        # serialize to PK as string
+        return self.serialize_object(appstruct)
+
+    def serialize_object(self, obj):
+        """
+        Serialize the given object to its primary key as string.
+
+        Default logic assumes the object has a UUID; subclass can
+        override as needed.
+
+        :param obj: Object reference for the node.
+
+        :returns: Object primary key as string.
+        """
+        return obj.uuid.hex
 
     def deserialize(self, node, cstruct):
         """ """
@@ -417,7 +430,7 @@ class ObjectRef(colander.SchemaType):
         if 'values' not in kwargs:
             query = self.get_query()
             objects = query.all()
-            values = [(obj.uuid.hex, str(obj))
+            values = [(self.serialize_object(obj), str(obj))
                       for obj in objects]
             if self.empty_option:
                 values.insert(0, self.empty_option)