Use update_column to set :last_activity_at for project

app/observers/project_activity_cache_observer.rb

@@ -2,7 +2,7 @@ class ProjectActivityCacheObserver < BaseObserver
   observe :event
 
   def after_create(event)
-    event.project.update_attribute(:last_activity_at, event.created_at) if event.project
+    event.project.update_column(:last_activity_at, event.created_at) if event.project
   end
 end