Way to pick the best promotion between order adjustments and line item adjustments in Spree 2.2+

order_updater_decorator.rb

Spree::OrderUpdater.class_eval do
  
  module BetterPromotions

    # DD: called by 'update_adjustment_total' before setting totals on order
    def recalculate_adjustments
      # DD: first, calculate all_adjustments for Order, LineItem, and Shipment using Spree::ItemAdjustments
      super

      # DD: FYI: promo_totals are negative
      if order.line_items.sum(:promo_total) < order.adjustments.promotion.eligible.sum(:amount)
        
        # DD: if line items offer a better promo, make order adjustments ineligible
        order.adjustments.promotion.eligible.update_all(eligible:false)

      else
        
        # DD: if order has a better promo, make line item adjustments ineligible 
        #     and undo totals set by Spree::ItemAdjustments#update_adjustments
        order.all_adjustments.where(adjustable_type: 'Spree::LineItem').promotion.eligible.update_all(eligible:false)
        order.line_items.update_all("adjustment_total = additional_tax_total, promo_total = 0")

      end

    end

  end

  prepend BetterPromotions

end