Update RSpec for Product model

spec/models/product_spec.rb

@@ -21,4 +21,18 @@ RSpec.describe Product, type: :model do
     it { is_expected.to belong_to(:user) }
     it { is_expected.to belong_to(:category) }
   end
+
+  context 'custom methods' do
+    let!(:user) { create(:user) }
+    let!(:other_user) { create(:user) }
+    let!(:product) { create(:product, quantity: 3, user: other_user) }
+
+    it 'has valid in_stock?' do
+      expect(product.in_stock?(2)).to be_truthy
+    end
+
+    it 'has valid belongs_to_user?' do
+      expect(product.belongs_to_user?(user)).not_to be_truthy
+    end
+  end
 end