user.rb

def self.from_omniauth(auth)
  @user = User.find(self.id)
  unless User.where(provider: auth.provider, uid: auth.uid).first
    @user.provider = auth.provider
    @user.uid = auth.uid
    @user.save
  end
end

Users::OmniauthCallbacksController.rb

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

    if @user.persisted?
      sign_in_and_redirect @user, notice: "You have successfully connected Facebook."
    end
  end
end