to feeds over time, which means I have to store one "row" for each feed item for a long time (perhaps my own version of Yucca mountain ). The Map() itself is very simple. Here's the Python code : def RemoveOldFeedEntryRecordPropertiesMapper(feed_entry_record): OLD_PROPERTIES = ('entry_id_hash', 'entry_id') for name in OLD_PROPERTIES: if hasattr(feed_entry_record, name):