Fix page_part creation when design is changed

app/models/page.rb

@@ -29,7 +29,7 @@ class Page < Item
   protected
   
   def create_parts
-    if self.new_record? || self.design.changed?
+    if self.new_record? || self.design_id_changed?
       page_design = self.design
       parent = self.parent
       menu_part = parent.page_parts.detect{|page_part| page_part.kind.eql?('public_r_tag') && page_part.public_r_tag.eql?('sub_menu') && page_part.public_r_tag_object_id.eql?(parent.id.to_s)} if parent