Ruby on Rails with GNU Emacs

(autoload 'ruby-mode "ruby-mode" "Load ruby-mode")
(add-to-list 'auto-mode-alist '("\\.rb\\'" . ruby-mode))

(defun try-complete-abbrev (old)
(if (expand-abbrev) t nil))

(setq hippie-expand-try-functions-list
      '(try-complete-abbrev
        try-complete-file-name
        try-expand-dabbrev))

(require 'rails)

Using ri-emacs for read rdoc documents 

;; If you use cocoa emacs, you need set darwinports path.
(setenv "PATH"
	(concat "/opt/local/bin/:/usr/local/bin:"
		(getenv "PATH")))


(setf rails-api-root "/opt/local/lib/ruby/gems/1.8/doc/")

(setq ri-ruby-script "/path_to_install_ri_emacs/ri-emacs.rb")
(autoload 'ri "ri-ruby" nil t)

(add-hook 'ruby-mode-hook (lambda ()
			    (local-set-key "\C-h" 'ri)
			    (local-set-key "\M-\C-i" 'ri-ruby-complete-symbol)
			    (local-set-key "\C-c\C-h" 'ri-ruby-show-args)
			    ))

Using mmm-mode for edit rhtml files. 

(require 'mmm-mode)
(require 'mmm-auto)
(setq mmm-global-mode 'maybe)
(setq mmm-submode-decoration-level 2)
(set-face-background 'mmm-output-submode-face  "LightBlue")
(set-face-background 'mmm-code-submode-face    "LightGray")
(set-face-background 'mmm-comment-submode-face "LightYellow")
(set-face-background 'mmm-special-submode-face "Yellow")

(mmm-add-classes
 '((erb-code
    :submode ruby-mode
    :match-face (("<%#" . mmm-comment-submode-face)
                 ("<%=" . mmm-output-submode-face)
                 ("<%"  . mmm-code-submode-face))
    :front "<%[#=]?" 
    :back "%>" 
    :insert ((?% erb-code       nil @ "<%"  @ " " _ " " @ "%>" @)
             (?# erb-comment    nil @ "<%#" @ " " _ " " @ "%>" @)
             (?= erb-expression nil @ "<%=" @ " " _ " " @ "%>" @))
    )))

(mmm-add-classes
 '((gettext
    :submode gettext-mode
    :front "_(['\"]"
    :face mmm-special-submode-face
    :back "[\"'])")))

(mmm-add-classes
 '((html-script
    :submode javascript-mode
    :front "<script>"
    :back "</script>")))

(add-to-list 'auto-mode-alist '("\\.rhtml$" . html-mode))


(add-hook 'html-mode-hook
	  (lambda ()
	    (setq mmm-classes '(erb-code html-js html-script gettext embedded-css))
	    (mmm-mode-on)))

(add-to-list 'mmm-mode-ext-classes-alist '(ruby-mode nil gettext))

(global-set-key [f8] 'mmm-parse-buffer)

If you are using Emacs version 22, you need following patch to mmm-vars.el. 

diff -ru mmm-mode-0.4.8/mmm-vars.el mmm-mode-0.4.8.new/mmm-vars.el
--- mmm-mode-0.4.8/mmm-vars.el  2004-06-16 07:14:18.000000000 -0700
+++ mmm-mode-0.4.8.new/mmm-vars.el      2007-02-22 12:20:01.000000000 -0800
@@ -287,7 +287,16 @@
         c-type-prefix-key 
         comment-end 
         comment-start 
-        comment-start-skip))
+        comment-start-skip
+
+        c-block-prefix-charset
+        c-nonlabel-token-key
+        c-block-stmt-1-key
+        c-label-kwds-regexp
+        c-prefix-spec-kwds-re
+        c-decl-hangon-key
+        c-type-decl-suffix-key
+        ))
     ;; Skeleton insertion
     skeleton-transformation
     ;; Abbrev mode