Run Rails spec files inside Emacs, looking for fast specs first.

spec.el

(defun fast-spec-file(file)
  (gsub (gsub  file "app/" "spec/fast/") ".rb" "_spec.rb"))

(defun run-spec()
  "Runs the specs for the buffer where this is called.
The spec lookup works like this:
1. For foo.rb it'll look for spec/fast/[models|controllers|...]/foo_spec.rb
2. If (1) fails it will run `rspec-verify`."
  (interactive)
  (let* ((file-name (buffer-file-name))
        (fast-spec (fast-spec-file file-name)))
    (if
        (file-exists-p fast-spec) (rspec-run-single-file fast-spec)
       (rspec-verify))))

(defun gsub (string search-string replace &optional regexp-flag)
  "Replaces the occurences of `search-string` in `string` with `replace`."
  (with-temp-buffer
    (insert string)
    (goto-char (point-min))
    (let ((search-function (if regexp-flag 're-search-forward 'search-forward)))
      (while (funcall search-function search-string nil t)
        (replace-match replace))
      (buffer-string))))