fixed #1

app/helpers/spree/base_helper_decorator.rb

@@ -19,7 +19,7 @@ def active_for_sale_price product, sale_price
   end
 
   def supported_currencies_for_sale_price
-    supported_currencies || [ _current_currency ]
+    try(:supported_currencies) || [ _current_currency ]
   end
 
   private

spec/helpers/base_helper_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+
+RSpec.describe Spree::BaseHelper, type: :helper do
+  describe '#supported_currencies_for_sale_price' do
+    context '(chaining backwards compatibility)' do
+      it 'can fallback to hardcoded standard currency' do
+        expect(helper.supported_currencies_for_sale_price).to eq(['USD'])
+      end
+
+      it 'can fallback to config if present' do
+        allow(Spree::Config).to receive(:[]).with(:currency).and_return('CHF')
+        expect(helper.supported_currencies_for_sale_price).to eq(['CHF'])
+      end
+
+      it 'can fallback to current_currency if present (part of newer spree core)' do
+        allow(helper).to receive(:current_currency).and_return('CHF')
+        expect(helper.supported_currencies_for_sale_price).to eq(['CHF'])
+      end
+
+      it 'can use spree_multi_currency if present' do
+        allow(helper).to receive(:supported_currencies).and_return(['CHF'])
+        expect(helper.supported_currencies_for_sale_price).to eq(['CHF'])
+      end
+    end
+  end
+end

spree_sale_prices.gemspec

@@ -2,7 +2,7 @@
 Gem::Specification.new do |s|
   s.platform    = Gem::Platform::RUBY
   s.name        = 'spree_sale_prices'
-  s.version     = '1.0.0'
+  s.version     = '2.4.6'
   s.summary     = 'Adds sale pricing functionality to Spree Commerce'
   s.description = 'Adds sale pricing functionality to Spree Commerce. It enables timed sale planning for different currencies.'
   s.required_ruby_version = '>= 1.9.3'