Technically what you're saying is you can't use Rails, you'd have to use Ruby, because Rails is pre-existing code built on top of Ruby.
That also throws out jQuery and any of its libraries.
The beauty of hackathons is they force you to think about the deliverable and attempt to mitigate reinventing the wheel by using as many plugins as possible to deliver your concept.
Where do you draw the line? Let's say I'm a core developer on Rails, am I not allowed to use Rails then? What happens if I contributed 1 line of code to Rails, does that negate it then?
And obviated by the judges having no easy way to learn and isolate which "part" of the app they're testing, and how such compartmentalization goes strongly against human psychology.
I don't pretend it'll be "perfect", but it should let decisions fall (noisily) around what we think of as "fair" (since we're all running that assessment through the same hardware) which is really what we're looking for.
<rules-lawyering-participant>So I get my buddy/silent-partner to develop my cool and unique API beforehand and sneak it unannounced onto a relatively anonymous public Github account before the comp starts, but close enough to the start date that no-one else will have even heard of it yet.
I think there's a slight distinction between using Rails and entering into a hackathon with the intention to port a recently developed app into a 'mobile' version?
And you didn't write the Ruby(|node|php|python|golang) interpreter either, or gcc/clang that compiled the interpreter, or the kernel that gcc relies on. (or the cpu microcode - it's turtles all the way down).
Personally, I think the Upshot approach was a sleazy hack of the competition rules. On the other hand, with a $1 million prize, you'd have to be an idiot not to expect rules-lawyering to be at least as important in winning as a good idea…
That also throws out jQuery and any of its libraries.
The beauty of hackathons is they force you to think about the deliverable and attempt to mitigate reinventing the wheel by using as many plugins as possible to deliver your concept.