The good news is many companies are looking for Rails developers right now in Paris.
I suppose a few of them are OK with hiring junior developers and training them.
The bad news is, most french like to speak french, and startups/small businesses can't sponsor visas (I actually don't know the process but small companies just don't have time to fix all paperwork and such).
ParisRB has a job board: https://jobs.rubyparis.org/fr/job_offers
The most english-friendly offers are... not for Paris ^^; but try to apply to a few!
You'll get insight and if you do your homework right and post to french Google Groups or ParisRB's comments you may find companies willing to take you on board