Fix VoucherAdjustmentsService.calculate so we can call it mutiple time

It now uses `order.pre_discount_total` to make any calculation, that
means you can call it multiple time and you will still get the same
adjustment amount, included_tax and tax adjustment when needed. This
is assuming nothing changed on the order.

app/services/voucher_adjustments_service.rb

@@ -10,7 +10,7 @@ class VoucherAdjustmentsService
     # We only support one voucher per order right now, we could just loop on voucher_adjustments
     adjustment = order.voucher_adjustments.first
 
-    # Recalculate value
+    # Calculate value
     amount = adjustment.originator.compute_amount(order)
 
     # It is quite possible to have an order with both tax included in and tax excluded from price.
@@ -23,33 +23,35 @@ class VoucherAdjustmentsService
       handle_tax_included_in_price(order, amount)
     else
       adjustment.amount = amount
+      adjustment.save
     end
-
-    # Move to closed state
-    adjustment.close
   end
 
   def self.handle_tax_excluded_from_price(order, amount)
-    voucher_rate = amount / order.total
+    voucher_rate = amount / order.pre_discount_total
+
+    adjustment = order.voucher_adjustments.first
 
     # Adding the voucher tax part
     tax_amount = voucher_rate * order.additional_tax_total
 
-    adjustment = order.voucher_adjustments.first
     adjustment_attributes = {
-      amount: tax_amount,
       originator: adjustment.originator,
       order: order,
       label: "Tax #{adjustment.label}",
       mandatory: false,
-      state: 'closed',
+      state: 'open',
       tax_category: nil,
       included_tax: 0
     }
-    order.adjustments.create(adjustment_attributes)
+
+    # Update the amount if tax adjustment already exist, create if not
+    tax_adjustment = order.adjustments.find_or_initialize_by(adjustment_attributes)
+    tax_adjustment.amount = tax_amount
+    tax_adjustment.save
 
     # Update the adjustment amount
-    amount = voucher_rate * (order.total - order.additional_tax_total)
+    amount = voucher_rate * (order.pre_discount_total - order.additional_tax_total)
 
     adjustment.update_columns(
       amount: amount,
@@ -58,7 +60,7 @@ class VoucherAdjustmentsService
   end
 
   def self.handle_tax_included_in_price(order, amount)
-    voucher_rate = amount / order.total
+    voucher_rate = amount / order.pre_discount_total
     included_tax = voucher_rate * order.included_tax_total
 
     # Update Adjustment

spec/services/voucher_adjustments_service_spec.rb

@@ -22,7 +22,7 @@ describe VoucherAdjustmentsService do
       end
     end
 
-    context 'with price included in order price' do
+    context 'with tax included in order price' do
       subject { order.voucher_adjustments.first }
 
       let(:order) do
@@ -51,18 +51,14 @@ describe VoucherAdjustmentsService do
 
       it 'updates the adjustment included_tax' do
         # voucher_rate = amount / order.total
-        # -10 / 150 = -0.066666667
+        # -10 / 160 = -0.0625
         # included_tax = voucher_rate * order.included_tax_total
-        # -0.66666666 * 10 = -0.67
-        expect(subject.included_tax.to_f).to eq(-0.67)
-      end
-
-      it 'moves the adjustment state to closed' do
-        expect(subject.state).to eq('closed')
+        # -0.0625 * 10 = -0.625
+        expect(subject.included_tax.to_f).to eq(-0.63)
       end
     end
 
-    context 'with price not included in order price' do
+    context 'with tax not included in order price' do
       let(:order) do
         create(
           :order_with_taxes,
@@ -87,28 +83,34 @@ describe VoucherAdjustmentsService do
         VoucherAdjustmentsService.calculate(order)
       end
 
-      it 'includes amount withou tax' do
+      it 'includes amount without tax' do
         adjustment = order.voucher_adjustments.first
         # voucher_rate = amount / order.total
-        # -10 / 161 = -0.062111801
+        # -10 / 171 = -0.058479532
         # amount = voucher_rate * (order.total - order.additional_tax_total)
-        # -0.062111801 * (161 -11) = -9.32
-        expect(adjustment.amount.to_f).to eq(-9.32)
+        # -0.058479532 * (171 -11) = -9.36
+        expect(adjustment.amount.to_f).to eq(-9.36)
       end
 
       it 'creates a tax adjustment' do
         # voucher_rate = amount / order.total
-        # -10 / 161 = -0.062111801
+        # -10 / 171 = -0.058479532
         # amount = voucher_rate * order.additional_tax_total
-        # -0.0585 * 11 = -0.68
+        # -0.058479532 * 11 = -0.64
         tax_adjustment = order.voucher_adjustments.second
-        expect(tax_adjustment.amount.to_f).to eq(-0.68)
+        expect(tax_adjustment.amount.to_f).to eq(-0.64)
         expect(tax_adjustment.label).to match("Tax")
       end
 
-      it 'moves the adjustment state to closed' do
-        adjustment = order.voucher_adjustments.first
-        expect(adjustment.state).to eq('closed')
+      context "when re calculating" do
+        it "updates the tax adjustment" do
+          order.update_columns(item_total: 200)
+          tax_adjustment = order.voucher_adjustments.second
+
+          expect do
+            VoucherAdjustmentsService.calculate(order)
+          end.to change { tax_adjustment.reload.amount }
+        end
       end
     end