Sometimes when using Devise, we want to disable email/password authentication, and enable only omniauth. Here’s how we can do it:

1. Initial setup # 

rails new omnify -d=postgresql
cd omnify
rails db:setup

rails g controller static_pages landing_page

routes.rb

  root 'static_pages#landing_page'

bundle add devise
rails g devise:install
rails generate devise User
rails db:migrate

application.html.erb

  <%= notice %>
  <%= alert %>
  <% if current_user %>
    <%= link_to current_user.email, edit_user_registration_path %>
    <%= link_to "Log out", destroy_user_session_path, method: :delete %>
  <% else %>
    <%= link_to "Log in", new_user_session_path %>
    <%= link_to "Register", new_user_registration_path %>
  <% end %>

2. Add login with github #

3. Finally, allow login ONLY with social (github in our case) #

Devise Wiki - only omniauth

Disable unused devise routes in routes.rb:

  devise_for :users, controllers: {omniauth_callbacks: "users/omniauth_callbacks"},
    skip: [:sessions, :registrations]

Disable unused devise resources in user.rb:

devise :database_authenticatable,
       #:registerable,
       #:recoverable,
       :rememberable,
       :validatable,
       :omniauthable, omniauth_providers: [:github]

This will disable the users/sign_up routes.

users/sign_in will still work. Existing users will still see a field to sign in.

You might want to generate the devise views for users/sessions/new and remove the login form, if you want only social login:

rails generate devise:views -v sessions

4. BONUS. Optionally: Modify generated devise resources in routes.rb. #

Now you have the following routes:

rails routes | grep devise

new_user_session GET          /users/sign_in(.:format)    devise/sessions#new
user_session POST             /users/sign_in(.:format)    devise/sessions#create
destroy_user_session DELETE   /users/sign_out(.:format)   devise/sessions#destroy

That translate to:

routes.rb

  devise_for :users, controllers: {omniauth_callbacks: "users/omniauth_callbacks"},
    skip: [:sessions, :registrations]

  # as :user do
  devise_scope :user do
    get "/users", to: "devise/sessions#new", as: :new_user_session
    post "/users/sign_in", to: "devise/sessions#create", as: :user_session
    delete "/users/sign_out", to: "devise/sessions#destroy", as: :destroy_user_session
  end

You might want to disable the get route to remove users/sign_in - Not really recommended.

Using devise_for