devise_invitable with omniauthable

invitations_controller.rb

class InvitationsController < Devise::InvitationsController

  # GET /resource/invitation/accept?invitation_token=abcdef
  def edit
    if params[:invitation_token] && self.resource = resource_class.to_adapter.find_first( :invitation_token => params[:invitation_token] )
      session[:invitation_token] = params[:invitation_token]
      render :edit
    else
      set_flash_message(:alert, :invitation_token_invalid)
      redirect_to after_sign_out_path_for(resource_name)
    end
  end

  # PUT /resource/invitation
  def update
    self.resource = resource_class.accept_invitation!(params[resource_name])

    if resource.errors.empty?
      session[:invitation_token] = nil
      set_flash_message :notice, :updated
      sign_in(resource_name, resource)
      respond_with resource, :location => after_accept_path_for(resource)
    else
      respond_with_navigational(resource){ render :edit }
    end
  end

omniauth_callbacks_controller.rb

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController
  def facebook/twitter/whatever
    auth = request.env["omniauth.auth"]
    identity = #find user by omniauth provider and uid
    if identity
      #do all regular sign in stuff here
    else
      #no identity found
      token = session[:invitation_token]
      if token && user = User.where(:invitation_token => token).first
        #some app specific code
      else
        user = User.new
      end

      user.apply_omniauth(auth)
      if user.valid?
        if user.invited?
          user.accept_invitation! 
        else
          user.save
        end
        sign_in(:user, user)
        redirect_to user_url(user)
      else
        #app specific code
      end
    end

    session[:invitation_token] = nil
  end

routes.rb

  devise_for :users,
    :controllers => {
      :invitations => 'invitations',
      :omniauth_callbacks => 'users/omniauth_callbacks'
    }