im trying redirect_to :back after the captcha is valid, but i think its going back in history. so basically i need to find a way to recapture the url that called the check captcha function.
redirect_to :back should do exactly what you need - it works on information contained in http referef header, so it should go back to the page which originated the request. From the source:
when :back request.env["HTTP_REFERER"] ? redirect_to(request.env["HTTP_REFERER"]) : raise(RedirectBackError)