Skip subunit write for non-attributes on currency change in getter

lib/money-rails/active_record/monetizable.rb

@@ -205,7 +205,7 @@ def read_monetized(name, subunit_name, options = nil, *args, **kwargs)
             result = memoized
           else
             memoized_amount = memoized.amount.to_money(attr_currency)
-            write_attribute subunit_name, memoized_amount.cents
+            write_attribute subunit_name, memoized_amount.cents if has_attribute? subunit_name
             # Cache the value (it may be nil)
             result = instance_variable_set("@#{name}", memoized_amount)
           end

spec/active_record/monetizable_spec.rb

@@ -105,6 +105,14 @@ def assert_monetized_attributes(monetized_attributes, expected_attributes)
         expect(transaction.amount_cents).to eq(20000)
       end
 
+      it "update to instance currency field gets applied to converted methods" do
+        transaction = Transaction.create(amount: '200', tax: '10', currency: 'USD')
+        expect(transaction.total).to eq(Money.new(21000, 'USD'))
+
+        transaction.currency = 'CLP'
+        expect(transaction.total).to eq(Money.new(210, 'CLP'))
+      end
+
       it "raises an error if trying to create two attributes with the same name" do
         expect do
           class Product