Code For OmniAuth

omniauth_controller.rb

## omniauth_controller 

class OmniauthCallbacksController < Devise::OmniauthCallbacksController
  
 
  def facebook
    @auth = request.env["omniauth.auth"]
    @user ||= User.find_for_facebook_oauth(@auth)
    if @user.persisted?
      
      sign_in @user
      redirect_to home_path 
      set_flash_message(:notice, :success, :kind => "Facebook") if is_navigational_format?
    else
      session["devise.facebook_data"] = request.env["omniauth.auth"].except('extra')
      redirect_to home_path
    end
    
  end

  def destroy
    reset_session
    redirect_to root_url, :notice => 'Signed out!'
  end

  def failure
    reset_session
    redirect_to root_url, :alert => "Authentication error: #{params[:message]}"
  end

end

## models/user.rb 


class User < ActiveRecord::Base
  # Relations
  has_many :posts
  has_one :profile

  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
  :recoverable, :rememberable, :trackable, :validatable,
  :omniauthable, omniauth_providers: [:facebook]

  # Validations
  # :email
  validates_format_of :email, with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i

  def self.find_for_facebook_oauth(auth)
    if user = User.find_by_email(auth.info.email)
      user
    else
      user = User.create(
        provider:   auth.provider,
        uid:        auth.uid,
        email:      auth.info.email,
        password:   Devise.friendly_token[0,20]
      )

      @username = '@' + auth.extra.raw_info.name.tr!(' ', '_').downcase
      user.create_profile(
        username:    @username,
        first_name:  auth.info.first_name,
        last_name:   auth.info.last_name,
        gender:      auth.extra.raw_info.gender,
        country:     auth.extra.raw_info.locale,
        image:       auth.info.image
      )

      if auth.info.image.present?
        user.profile.remote_image_url = auth.info.image
        user.profile.save!
      end

      if auth.extra.raw_info.username.present?
        user.profile.username = auth.extra.raw_info.username
        user.profile.save!
      else
        user.profile.username = @username
      end
      user
    end
  end
  
end