Overriding Spree tax calculations

order_decorator.rb

# See http://guides.spreecommerce.com/logic_customization.html#extending-classes
# on where/how to use these

Spree::Order.class_eval do
  def shipping_tax_zone
    zone_address = ship_address
    Spree::Zone.match(zone_address) || Spree::Zone.default_tax
  end

  def billing_tax_zone
    zone_address = bill_address
    Spree::Zone.match(zone_address) || Spree::Zone.default_tax
  end
end

tax_rate_decorator.rb

Spree::TaxRate.class_eval do
  # Gets the array of TaxRates appropriate for the specified order
  def self.match(order)
    return [] unless order.tax_zone
    all.select do |rate|
      rate.zone == order.tax_zone || rate.zone.contains?(order.shipping_tax_zone) || rate.zone.contains?(order.billing_tax_zone) || rate.zone.default_tax
    end
  end
end