yeah, source as in models.Source. and the `in` query sounds good. just fyi, `in` queries aren't handled in the datastore query engine itself; they're implemented purely client side as a fan out to a single == query per filter value...but they all run in parallel, so the latency should improve at least.

that will probably change your code structure a fair amount, so feel free to add a TODO and punt it to a later PR.

btw, https://developers.google.com/appengine/docs/python/tools/appstats is a great way to visualize datastore operations, http fetches, and other parts of an app engine request. you can use it in dev_appserver by uncommenting the lines in `appengine_config.py` under "uncomment for app stats," running a request or two, then going to http://localhost:8080/_ah/stats/ .