let's use eglot or lsp-mode

Ruby setup

install solargraph via:

gem install solargraph

or add 'solargraph' in you Gemfile.

start solargraph server:

solargraph server

Emacs setup

Clone this repository somewhere and add this directory to your load-path.

basic setup:

(require 'solargraph)
(define-key ruby-mode-map (kbd "M-i") 'solargraph:complete)

Autocomplete setup

(require 'ac-solargraph)
(define-key ruby-mode-map (kbd "M-i") 'ac-solargraph:complete)

Solargraph project: https://github.com/castwide/solargraph