Ensure variants don't end up with invalid data when a product's variant_unit is changed

Fixes an issue where a product's variant_unit value is changed from "weight" to "items" and some of the product's variants can be left in an invalid state, which in turn breaks cloning of order cycles (with fatal errors).

app/models/spree/variant.rb

@@ -65,6 +65,7 @@ module Spree
 
     before_validation :set_cost_currency
     before_validation :update_weight_from_unit_value, if: ->(v) { v.product.present? }
+    before_validation :ensure_unit_value
 
     after_save :save_default_price
     after_save :update_units
@@ -297,5 +298,11 @@ module Spree
       exchange_variants(:reload).destroy_all
       yield
     end
+
+    def ensure_unit_value
+      return unless product&.variant_unit == "items" && unit_value.nil?
+
+      self.unit_value = 1.0
+    end
   end
 end

spec/models/spree/variant_spec.rb

@@ -783,4 +783,17 @@ module Spree
       expect(e.reload.variant_ids).to be_empty
     end
   end
+
+  describe "#ensure_unit_value" do
+    let(:product) { create(:product, variant_unit: "weight") }
+    let(:variant) { create(:variant, product_id: product.id) }
+
+    context "when a product's variant_unit value is changed from weight to items" do
+      it "sets the variant's unit_value to 1" do
+        product.update(variant_unit: "items")
+
+        expect(variant.unit_value).to eq 1
+      end
+    end
+  end
 end