This is one thing I'm a little concerned about leaving as a TODO, since it seems like it could be a little difficult to change later.

One question: it could be that you are doing original-post-discovery on a twitter post, and find rel=syndication links to facebook posts. Is it possible to look up a Source given the author's domain_url and the silo url of the syndicated content? maybe `Source.query(domain_url=...)`, and then filter manually by checking `source.AS_CLASS.DOMAIN`?

And I think I basically understand the difference between adding a source property to SyndicatedPost and making SyndicatedPost a child of the Source, but not the tradeoffs of one approach over the other. I'd be very open to an executive decision if you have a preference for one or the other ;)