Validate a polymorphic association for a new record

validates_polymorphic.rb

class Address < ActiveRecord::Base
  belongs_to :addressable, :polymorphic => true

  #use these instead of validates_presence_of :addressable
  validates_presence_of :addressable_type
  validates_presence_of :addressable_id, :unless => Proc.new { |a|
    #if it's a new record and addressable is nil and addressable_type is set
    #   then try to find the addressable object in the ObjectSpace
    #       if the addressable object exists, then we're valid;
    #       if not, let validates_presence_of do it's thing
    if (new_record? && !addressable && addressable_type)
      addressable = nil
      ObjectSpace.each_object(addressable_type.constantize) do |o|
        addressable = o if o.addresses.include?(a) unless addressable
      end
    end
    addressable
  }
end