Speed up add to cart: Update the order once per fee calculation, rather than for every line item x fee

app/models/spree/adjustment_decorator.rb

@@ -35,5 +35,19 @@ module Spree
     def display_included_tax
       Spree::Money.new(included_tax, { :currency => currency })
     end
+
+    def self.without_callbacks
+      skip_callback :save, :after, :update_adjustable
+      skip_callback :destroy, :after, :update_adjustable
+
+      result = yield
+
+    ensure
+      set_callback :save, :after, :update_adjustable
+      set_callback :destroy, :after, :update_adjustable
+
+      result
+    end
+
   end
 end

app/models/spree/order_decorator.rb

@@ -164,21 +164,29 @@ Spree::Order.class_eval do
 
   def update_distribution_charge!
     with_lock do
-      EnterpriseFee.clear_all_adjustments_on_order self
 
-      line_items.each do |line_item|
-        if provided_by_order_cycle? line_item
-          OpenFoodNetwork::EnterpriseFeeCalculator.new.create_line_item_adjustments_for line_item
+      # Without intervention, the Spree::Adjustment#update_adjustable callback is called
+      # once for every (line item x fee), which triggers a costly Spree::Order#update!
+      Spree::Adjustment.without_callbacks do
+        EnterpriseFee.clear_all_adjustments_on_order self
 
-        else
-          pd = product_distribution_for line_item
-          pd.create_adjustment_for line_item if pd
+        line_items.each do |line_item|
+          if provided_by_order_cycle? line_item
+            OpenFoodNetwork::EnterpriseFeeCalculator.new.create_line_item_adjustments_for line_item
+
+          else
+            pd = product_distribution_for line_item
+            pd.create_adjustment_for line_item if pd
+          end
+        end
+
+        if order_cycle
+          OpenFoodNetwork::EnterpriseFeeCalculator.new.create_order_adjustments_for self
         end
       end
 
-      if order_cycle
-        OpenFoodNetwork::EnterpriseFeeCalculator.new.create_order_adjustments_for self
-      end
+      # After fees are calculated, we update the order once
+      update!
     end
   end
 

spec/models/spree/order_spec.rb

@@ -54,6 +54,8 @@ describe Spree::Order do
       product_distribution.should_receive(:create_adjustment_for).with(line_item)
       subject.stub(:product_distribution_for) { product_distribution }
 
+      subject.should_receive(:update!)
+
       subject.update_distribution_charge!
     end
 
@@ -65,6 +67,8 @@ describe Spree::Order do
 
       subject.stub(:product_distribution_for) { nil }
 
+      subject.should_receive(:update!)
+
       subject.update_distribution_charge!
     end
 
@@ -90,6 +94,8 @@ describe Spree::Order do
       OpenFoodNetwork::EnterpriseFeeCalculator.any_instance.stub(:create_order_adjustments_for)
       subject.stub(:order_cycle) { order_cycle }
 
+      subject.should_receive(:update!)
+
       subject.update_distribution_charge!
     end