address.rb

class Address < ActiveRecord::Base

  belongs_to :customer

  validates_presence_of :line_1, :city, :postcode
  validates_format_of :tag, :with => MVOR::Patterns.dashed_word, :allow_blank => true

  belongs_to :addressable, :polymorphic => true

  def expendable?
    # pending
    true
  end

end

customer.rb

class Customer < User
  
  # The rest of the `user` attributes are implicitly protected by 
  # an explicit declaration of accessible attributes in the 
  # User class.
  attr_accessible :business, :account_address, :addresses

  has_many :orders
  
  has_many :addresses, :as => :addressable
  accepts_nested_attributes_for :addresses
  
  has_one  :account_address, :as => :addressable, :class_name => 'Address', :conditions => ['tag = ?','account-address']
  accepts_nested_attributes_for :account_address

  # ..... snip .....

end