class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
def facebook

@user = User.from_omniauth(request.env["omniauth.auth"])

if @user.persisted?
  byebug
  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
def failure
  redirect_to root_path
 end
end

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

Devise.setup do |config|
config.omniauth :facebook, ENV['APP_ID'], ENV['APP_SECRET'],
              callback_url: ENV['CALLBACK_URL'],
              scope: 'email

user.rb
devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable
devise :omniauthable, :omniauth_providers => [:facebook]

def self.from_omniauth(auth)
  where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
    user.email = auth.info.email
    user.password = Devise.friendly_token[0,20]
  end
end

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.find_for_facebook_oauth(access_token)
  if user = User.where(:url => access_token.info.urls.Facebook).first
    user
  else
    User.create!(:provider => access_token.provider, :url => access_token.info.urls.Facebook, :username => access_token.extra.raw_info.name, :nickname => access_token.extra.raw_info.username, :email => access_token.extra.raw_info.email, :password => Devise.friendly_token[0,20])
  end
end

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :facebook, ENV['APP_ID'], ENV['APP_SECRET']
end