Add tax_category_id to Product model

The LineItem and the Variant are referencing the tax_category_id, but does not have the same logic as the tax_category (the fallback to the default tax category). This method mirrors the same logic as the tax_category method.

Author: sascha-karnatz

core/app/models/spree/product.rb

@@ -147,6 +147,11 @@ def tax_category
       super || Spree::TaxCategory.find_by(is_default: true)
     end
 
+    # @return [Integer] tax category id for this product, or the default tax category id
+    def tax_category_id
+      super || tax_category&.id
+    end
+
     # Ensures option_types and product_option_types exist for keys in
     # option_values_hash.
     #

core/spec/models/spree/product_spec.rb

@@ -285,6 +285,35 @@ class Extension < Spree::Base
       end
     end
 
+    describe "#tax_category_id" do
+      context "when the product has a tax category" do
+        let(:tax_category) { create(:tax_category) }
+        let(:product) { create(:product, tax_category:) }
+
+        it "returns the product's tax category id" do
+          expect(product.tax_category_id).to eq(tax_category.id)
+        end
+      end
+
+      context "when the product has no tax category" do
+        let(:product) { create(:product, tax_category: nil) }
+
+        context "and a default tax category exists" do
+          let!(:default_tax_category) { create(:tax_category, is_default: true) }
+
+          it "returns the default tax category's id" do
+            expect(product.tax_category_id).to eq(default_tax_category.id)
+          end
+        end
+
+        context "and no default tax category exists" do
+          it "returns nil" do
+            expect(product.tax_category_id).to be_nil
+          end
+        end
+      end
+    end
+
     context "variants_and_option_values_for" do
       let!(:high) { create(:variant, product:) }
       let!(:low) { create(:variant, product:) }