Bulk fetch from Datastore only if objects aren't already in the cache

cache_and_saver_pattern.py

# Here is a more complicated endpoint that retrieve objects
# from datastore in multiple phases, and build up a queue of objects
# to bulk commit at the end

from pymacaron.auth import get_userid

def do_get_recommended_new_friends(data):
  
  cache = ObjectCache()
  saver = ObjectSaver(cache=cache)
  
  # Retrieve the viewer's profile, as well as some other profiles whose
  # IDs are in data.user_ids
  viewer_id = get_userid()
  cache.fetch_missing_objects([viewer_id] + data.user_ids)
   
  # Queue up an update of the viewer's profile
  viewer = cache.get_object(viewer_id)
  viewer.update_some_things()
  saver.put_object(viewer)
  
  # Query Elasticsearch for new users that the viewer should see in her
  # friend's feed
  suggested_friend_ids = es.search_new_friends()
  

  # Now, we need to get the profiles of those new friends from Datastore. But
  # what if we already have them in the cache? We should let the cache do the
  # Datastore bulk get, after first filtering out the profiles that are already
  # in the cache:
  cache.fetch_missing_objects(suggested_friend_ids)
  
  # And queue up more things to bulk insert
  for o in get_objects_to_save():
    saver.put_object(o)
    
  # Finally, bulk insert all these objects:
  saver.commit()