gistfile1.rb

class Address < ActiveRecord::Base
  belongs_to :addressable, :polymorphic => true
  named_scope :default, :conditions => { :default => true }
  named_scope :without, lambda{|address| { :conditions => ["id <> ?", address.to_param] }}

  before_save :set_default

  private

  def set_default
    return true unless addressable.kind_of? User

    if default?
      addressable.addresses.default.without(self).each{|a| a.update_attribute(:default, false) }
    else
      default = true if new_record? && addressable.addresses.default.empty?
    end
  end
end