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# app/models/user.rb
 def self.new_with_session(params, session)
    super.tap do |user|
      if data = session["devise.facebook_data"] && session["devise.facebook_data"]["extra"]["raw_info"]
        user.email = data["email"] if user.email.blank?
      end
    end
  end

  def self.from_omniauth(auth)

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# app/controllers/users/omniauth_callbacks_controller.rb
class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook
    # You need to implement the method below in your model (e.g. app/models/user.rb)
    @user = User.from_omniauth(request.env["omniauth.auth"])

    if @user.persisted?
      sign_in_and_redirect @user, :event => :authentication #this will throw if @user is not activated
      set_flash_message(:notice, :success, :kind => "Facebook") if is_navigational_format?
    else

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# execute from terminal
rails g controller users/omniauth_callbacks

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# config/routes.rb
devise_for :users, controllers: { omniauth_callbacks: 'users/omniauth_callbacks' }

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<%-# app/views/cities/index.html.erb -%>
<% if user_signed_in? %>
  <%= link_to('Logout', destroy_user_session_path, :method => :delete) %>        
<% else %>
  <%= link_to('Login', new_user_session_path)  %>  
<% end %>

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#execute from terminal
bundle install

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# app/models/user.rb
devise :omniauthable, omniauth_providers: [:facebook]

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# Gemfile.rb
gem 'omniauth-oauth2', '1.3.1'
gem 'omniauth-facebook'

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#  config/routes.rb
root 'cities#index'

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# app/controllers/application_controller.rb
before_action :authenticate_user!