Thanks for checking out the example and I agree that it may be overly trivial. We do plan to investigate this further and will blog about it at https://starthq.com/blog/

I really think that depending on one's requirements it should be possible to query relational database using MongoDB syntax and use (basic) relational queries on Mongo data.

There have been tools like Metrica (https://starthq.com/apps/metrica) that do this, but we're working on implementing that as a Node package.