Login with facebook

Step 1:

Add gem into gemfile

gem 'omniauth-facebook'

Step 2:

Facebook App setup

https://developers.facebook.com/

Step 3:

run command on terminal

bundle install

Step 4:
Need to add facebook app id and facebook secret key

config/initializers/devise.rb

config.omniauth :facebook, 'facebook app id', 'facebook app secret',{:scope => 'email'}

Step 5:

we will need to create a User model to store our information that Facebook returns. Run the commands below to create the user model.

rails g model user provider uid name oauth_token oauth_expires_at:datetime
rake db:migrate

Step 6:
create omniauth_callbacks_controller

devise/omniauth_callbacks_controller.rb

class Devise::OmniauthCallbacksController < ApplicationController

  skip_before_filter :authenticate_user!

  def facebook

    begin

      @user = User.from_facebook_omniauth(request.env['omniauth.auth'])

      session[:user_id] = @user.id

      sign_in @user

      flash[:success] = "Welcome, #{@user.name}!"

    rescue

      flash[:warning] = "There was an error while trying to authenticate you..."

    end

    redirect_to users_path

  end



  def failure

    redirect_to root_path

  end

end

Step 7:

  config/routes.rb

devise_for :users, :controllers => {:omniauth_callbacks => "devise/omniauth_callbacks",registrations:"devise/registrations"}

Step 8:

Write a method in user model
app/model/user.rb

 def self.from_facebook_omniauth(auth)

    email = auth.info.email.present? ? auth.info.email : "#{auth.uid}@facebook.com"

     user_exist = User.find_by_email(email)

     if user_exist.nil?

       where(provider: auth.provider, uid: auth.uid).first_or_create do |user|

         user.provider = auth.provider

         user.uid = auth.uid

         user.email = email

         user.first_name = auth.info.name.split(' ')[0]

         user.last_name = auth.info.name.split(' ')[1]

         user.username = auth.info.name.split(' ')[0].downcase

         user.password = Devise.friendly_token[0,20]

       end

     else

       user_exist.update(provider: auth.provider, access_token: auth.credentials.token, uid: auth.uid, first_name: auth.info.name, last_name: nil)

     end

     user_exist = User.find_by_email(email)

   end