Introduce Spree::ItemAdjustments class

app/models/spree/item_adjustments.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+module Spree
+  # Manage (recalculate) item (LineItem or Shipment) adjustments
+  class ItemAdjustments
+    attr_reader :item
+
+    delegate :adjustments, :order, to: :item
+
+    def initialize(item)
+      @item = item
+    end
+
+    def update
+      update_adjustments if item.persisted?
+      item
+    end
+
+    def update_adjustments
+      adjustment_total = adjustments.map(&:update!).compact.sum
+
+      item.update_columns(
+        adjustment_total: adjustment_total,
+        updated_at: Time.zone.now
+      )
+    end
+  end
+end