class Invoice < ActiveRecord::Base
    
  has_many :line_items
  has_many :payments

  calculated_attr :subtotal, :dependent => [ :line_items ] do
    line_items.inject(0) { |sum, line| sum + line.total }
  end
  
  calculated_attr :paid, :dependent => [ :payments ] do
    payments.inject(0) { |sum, payment| sum + payment.total }
  end
  
  calculated_attr :total, :dependent => [ :line_items ] do
    subtotal + tax
  end
  
  calculated_attr :due, :dependent => [ :line_items, :payments ] do
    total - paid
  end

end

class LineItem < ActiveRecord::Base
end

class Payment < ActiveRecord::Base
end