Simple Spree adjusment

line_item_decorator.rb

#
# Decorate Spree::LineItem model to add keg deposits
# and CRV automatically to the order (derived from
# the package format fees) when line items are changed
#
Spree::LineItem.class_eval do

  after_save :update_order_package_fees

  def update_order_package_fees
    order.update_package_fees!
  end

end

order_decorator.rb

#
# Decorate Spree::Order model with method to calculate 
# keg deposits and CRV automatically to the order (derived
# from the package format fees)
#
Spree::Order.class_eval do

  # Update custom fees derived from the package format (such as CRV for
  # cases or keg deposit for kegs - see PackageFormat model)
  def update_package_fees!
    adjustments.where(:source_type => 'package-format-fee').destroy_all

    fees = {}

    self.line_items.each do |line_item|
      qty = line_item.quantity
      fee_name = line_item.product.package_format.fee_name
      fee_value = line_item.product.package_format.fee_value
      fees[fee_name] ||= 0
      fees[fee_name] += qty * fee_value
    end

    fees.each do |(fee_name, fee_value)|
      add_package_fee(fee_name, fee_value) if fee_value > 0
    end

    update!
  end

  private
    def add_package_fee(label, amount)
      self.adjustments.create({
        :amount => amount,
        :adjustable => self,
        :source_type => 'package-format-fee',
        :label => label,
        :mandatory => true,
        :locked => true
      }, :without_protection => true)
    end

end