Use OrderTaxAdjustmentsFetcher service

app/services/order_tax_adjustments_fetcher.rb

@@ -0,0 +1,28 @@
+# This class will be used to get Tax Adjustments related to an order,
+# and proceed basic calcultation over them.
+
+class OrderTaxAdjustmentsFetcher
+  def initialize(order)
+    @order = order
+  end
+
+  def totals
+    all.each_with_object({}) do |adjustment, hash|
+      tax_rates = TaxRateFinder.tax_rates_of(adjustment)
+      tax_rates_hash = Hash[tax_rates.collect do |tax_rate|
+        tax_amount = tax_rates.one? ? adjustment.included_tax : tax_rate.compute_tax(adjustment.amount)
+        [tax_rate, tax_amount]
+      end]
+      hash.update(tax_rates_hash) { |_tax_rate, amount1, amount2| amount1 + amount2 }
+    end
+  end
+
+  private
+
+  attr_reader :order
+
+  def all
+    order.adjustments.with_tax +
+      order.line_items.includes(:adjustments).map { |li| li.adjustments.with_tax }.flatten
+  end
+end