gistfile1.rb

class BuildingsController < ApplicationController
  def destroy
    @building = Building.find params[:id]

    begin
      @building.destroy
    rescue
      flash[:notice] = $!.message
    end
    
    respond_to do |format|
      format.html { redirect_to buildings_url } 
      format.xml  { head :ok } 
    end
  end
end

class Building < ActiveRecord::Base
  before_destroy :require_no_rooms
  has_many :rooms

  private

  def require_no_rooms
    self.errors.add :base, 'A building can not be deleted if it contains rooms.'
    raise ActiveRecord::RecordInvalid.new self unless rooms.count == 0
  end
end