Avoid re-creating tax adjustments

Previously, ItemAdjuster#adjust! would always destroy all tax
adjustments on the item and recreate them. This was unnecessarily slow
in the common case of needing no changes (especially following solidusio#1479).

This changes ItemAdjuster#adjust! to instead update the adjustment for
a rate if it already exists, create adjustments if they are missing for
a matching rate, and destroy adjustments for rates which are no longer
matching.

core/app/models/spree/tax/item_adjuster.rb

@@ -20,12 +20,31 @@ def initialize(item, options = {})
         @rates_for_default_zone = options[:rates_for_default_zone]
       end
 
-      # Deletes all existing tax adjustments and creates new adjustments for all
-      # (geographically and category-wise) applicable tax rates.
+      # This updates the amounts for adjustments which already exist and
+      # creates and remove adjustments as needed to match the applicable
+      # (geographically and category-wise) tax rates.
       def adjust!
-        item.adjustments.destroy(item.adjustments.select(&:tax?))
+        rates = rates_for_item(item)
 
-        rates_for_item(item).each { |rate| rate.adjust(nil, item) }
+        tax_adjustments = item.adjustments.select(&:tax?)
+        active_adjustments = rates.map do |rate|
+          # Find an existing adjustment from the same source.
+          # All tax adjustments already have source_type == 'Spree::TaxRate' so
+          # we need only check source_id.
+          adjustment = tax_adjustments.detect{|a| a.source_id == rate.id }
+          if adjustment
+            adjustment.update!
+            adjustment
+          else
+            # Create a new adjustment
+            rate.adjust(nil, item)
+          end
+        end
+
+        unmatched_adjustments = tax_adjustments - active_adjustments
+
+        # Remove any tax adjustments tied to rates which no longer match
+        item.adjustments.destroy(unmatched_adjustments)
       end
     end
   end