Make weight calculator compute 0 for variants with unit different from weight

app/models/calculator/weight.rb

@@ -16,6 +16,8 @@ module Calculator
       total_weight(line_items) * preferred_per_kg
     end
 
+    private
+
     def total_weight(line_items)
       line_items.sum do |line_item|
         line_item_weight(line_item)
@@ -23,6 +25,7 @@ module Calculator
     end
 
     def line_item_weight(line_item)
+      return 0 if line_item.variant.product.andand.variant_unit != 'weight'
       if line_item.final_weight_volume.present?
         # Divided by 1000 because grams is the base weight unit and the calculator price is per_kg
         line_item.final_weight_volume / 1000

spec/models/calculator/weight_spec.rb

@@ -29,10 +29,16 @@ describe Calculator::Weight do
     end
 
     describe "and with final_weight_volume defined" do
+      before { line_item.update_attribute :final_weight_volume, '18000' }
+
       it "computes fee using final_weight_volume, not the variant weight" do
-        line_item.final_weight_volume = "18000"
         expect(subject.compute(line_item)).to eq(10 * 18)
       end
+
+      it "returns zero for variant where unit type is not weight" do
+        line_item.variant.product.update_attribute :variant_unit, 'items'
+        expect(subject.compute(line_item)).to eq(0)
+      end
     end
   end