I replaced the stock implementation of ruby-default-ri-entry with the
following code so that I could use it even if I didn’t have a current
word.

(defun ruby-default-ri-entry ()
  (let (word)
    (save-excursion
      (setq word (or (current-word) ""))
      (if (string-match "[._]+$" word)
	  (setq word (substring word 0 (match-beginning 0))))
      (format "%s%s"
	      word
	      ""))))

../emacs/ruby-config.el