Devise.setup do |config|
  config.omniauth :facebook, ENV["FB_ID"], ENV["FB_SECRET"], scope: 'email', info_fields: 'email, first_name,last_name', image_size: 'large'
end

Rails.application.routes.draw do
  devise_for :users, controllers: { omniauth_callbacks: 'users/omniauth_callbacks' }
end

# app/models/user.rb

class User < ActiveRecord::Base
  devise :omniauthable, omniauth_providers: [:facebook]
end

$ rails g migration AddOmniauthToUsers
    provider uid picture first_name last_name token token_expiry:datetime
$ rake db:migrate

# app/models/user.rb
class User < ActiveRecord::Base
  def self.find_for_facebook_oauth(auth)
    where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
      user.provider = auth.provider
      user.uid = auth.uid
      user.email = auth.info.email
      user.password = Devise.friendly_token[0,20]  # Fake password for validation
      user.first_name = auth.info.first_name
      user.last_name = auth.info.last_name
      user.picture = auth.info.image
      user.token = auth.credentials.token
      user.token_expiry = Time.at(auth.credentials.expires_at)
    end
  end
end

# app/controllers/users/omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    user = User.find_for_facebook_oauth(request.env['omniauth.auth'])

    if user.persisted?
      sign_in_and_redirect user, event: :authentication
      set_flash_message(:notice, :success, kind: 'Facebook') if is_navigational_format?
    else
      session['devise.facebook_data'] = request.env['omniauth.auth']
      redirect_to new_user_registration_url
    end
  end
end

/app/app/controllers/Users/omniauth_callbacks_controller.rb:1:in `<top (required)>': uninitialized constant Users (NameError)
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:472:in `block (2 levels) in eager_load!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:471:in `each'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:471:in `block in eager_load!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:469:in `each'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:469:in `eager_load!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/engine.rb:346:in `eager_load!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/finisher.rb:56:in `each'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application/finisher.rb:56:in `block in <module:Finisher>'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:30:in `instance_exec'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:30:in `run'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:55:in `block in run_initializers'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:431:in `each_strongly_connected_component_from'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:349:in `block in each_strongly_connected_component'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:347:in `each'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:347:in `call'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:347:in `each_strongly_connected_component'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:226:in `tsort_each'
    from /app/vendor/ruby-2.3.1/lib/ruby/2.3.0/tsort.rb:205:in `tsort_each'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/initializable.rb:54:in `run_initializers'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application.rb:352:in `initialize!'
    from /app/config/environment.rb:5:in `<top (required)>'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/application.rb:328:in `require_environment!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:142:in `require_application_and_environment!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:67:in `console'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
    from /app/vendor/bundle/ruby/2.3.0/gems/railties-4.2.6/lib/rails/commands.rb:17:in `<top (required)>'
    from /app/bin/rails:9:in `require'
    from /app/bin/rails:9:in `<main>'