Handle unit scale changes

As discussed, this is the desired behaviour. The current screen appears to do this, but fails to save the changes.

app/views/admin/products_v3/_variant_row.html.haml

@@ -19,7 +19,7 @@
       = f.hidden_field :unit_description
       -# todo: create a method for value_with_description
       = f.text_field :unit_value_with_description,
-          value: [number_with_precision(variant.unit_value, precision: nil,strip_insignificant_zeros: true), variant.unit_description].compact_blank.join(" "),
+          value: [number_with_precision((variant.unit_value || 1) / (variant.product.variant_unit_scale || 1), precision: nil, strip_insignificant_zeros: true), variant.unit_description].compact_blank.join(" "),
           'aria-label': t('admin.products_page.columns.unit_value'), required: true
     .field
       = f.label :display_as, t('admin.products_page.columns.display_as')

app/webpacker/controllers/variant_controller.js

@@ -57,7 +57,7 @@ export default class VariantController extends Controller {
     if (match) {
       let unit_value = parseFloat(match[1].replace(",", "."));
       unit_value = isNaN(unit_value) ? null : unit_value;
-      unit_value *= this.variantUnitScale.value ? this.variantUnitScale.value : 1;
+      unit_value *= this.variantUnitScale.value ? this.variantUnitScale.value : 1; // Normalise to default scale
 
       this.unitValue.value = unit_value;
       this.unitDescription.value = match[3];

spec/system/admin/products_v3/products_spec.rb

@@ -270,6 +270,18 @@ describe 'As an admin, I can manage products', feature: :admin_style_v3 do
           # New scale is visible immediately
           expect(page).to have_button "Unit", text: "1kg"
         end
+
+        click_button "Save changes"
+
+        expect(page).to have_content "Changes saved"
+        product_a.reload
+        expect(product_a.variant_unit).to eq "weight"
+        expect(product_a.variant_unit_scale).to eq 1000 # kg
+        expect(variant_a1.reload.unit_value).to eq 1000 # 1kg
+
+        within row_containing_name("Medium box") do
+          expect(page).to have_button "Unit", text: "1kg"
+        end
       end
 
       it "saves a custom item unit name" do