flashing notice from a model throws

I have a view (list_users) that calls destroy_user and I'd like to get
it to flash an error within the view. The code below works for the
raise, but if I comment it out and try the 2 lines below it, I get the
following error:

"undefined local variable or method `flash' for #<User:0x28fd3f4>"

Any ideas?

user_controller.rb:

  def destroy_user
    if request.post?
      user = User.find(params[:id])
      user.destroy
      flash[:notice] = "user has been deleted!"
    end
    redirect_to(:action => :list_users)
  end

user.rb (model):

def after_destroy
    if User.count.zero?
      raise "Can't delete the last user"
      # I'd like it to do this, but can't seem to make it work:
      # flash.now[:notice] = "Can't remove the last user"
      # redirect_to(:action => "list_users")
    end
  end