Allow nil to be set as default currency

lib/money-rails/configuration.rb

@@ -20,7 +20,7 @@ def configure
     # Configuration parameters
 
     def default_currency
-      Money::Currency.new(Money.default_currency)
+      Money::Currency.new(Money.default_currency) if Money.default_currency
     end
 
     # Set default currency of money library
@@ -35,7 +35,7 @@ def register_currency=(currency_options)
     end
 
     def set_currency_column_for_default_currency!
-      iso_code = default_currency.iso_code
+      iso_code = default_currency&.iso_code
       currency_column.merge! default: iso_code
     end
 

spec/active_record/monetizable_spec.rb

@@ -949,6 +949,27 @@ class SubProduct < Product
         expect(price.amount).not_to eq(value.amount)
       end
 
+      context 'without a default currency' do
+        let(:product) { OtherProduct.new }
+
+        before do
+          around do |example|
+            default_currency = MoneyRails::Configuration.default_currency
+            MoneyRails::Configuration.default_currency = nil
+
+            example.run
+
+            MoneyRails::Configuration.default_currency = default_currency
+          end
+        end
+
+        it "errors when" do
+          expect do
+            product.write_monetized :price, :price_cents, 10.5, false, nil, {}
+          end.to raise_error(Money::Currency::NoCurrency)
+        end
+      end
+
       describe "instance_currency_name" do
         it "updates instance_currency_name attribute" do
           product.write_monetized :sale_price, :sale_price_amount, value, false, :sale_price_currency_code, {}

spec/dummy/app/models/other_product.rb

@@ -0,0 +1,3 @@
+class OtherProduct < ActiveRecord::Base
+  monetize :price_cents
+end