omniauth_callbacks_controller.rb

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

  def twitter
    callback_from :twitter
  end

  def google_oauth2
    callback_from :google
  end

  def instagram
    callback_from :google
  end

  private

  def callback_from(provider)
    provider = provider.to_s

    @user = User.find_for_oauth(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 => provider.capitalize) if is_navigational_format?
    else
      session["devise.#{provider}_data"] = request.env["omniauth.auth"].except("extra")
      redirect_to new_user_registration_url
    end
  end
end