Adds logic to swap items in cart on currency change

app/controllers/spree/currency_controller.rb

@@ -1,14 +1,56 @@
 module Spree
   class CurrencyController < Spree::StoreController
+
     def set
-      currency = supported_currencies.find { |currency| currency.iso_code == params[:currency] }
+      @currency = supported_currencies.find { |currency| currency.iso_code == params[:currency] }
+      # make shure that we update the current order, so the currency change is reflected
+      if current_order
+        update_order!
+      end
       session[:currency] = params[:currency] if Spree::Config[:allow_currency_change]
       respond_to do |format|
-        format.json { render :json => !currency.nil? }
+        format.json { render json: !@currency.nil? }
         format.html do
-          redirect_to root_path
+          # We want to go back to where we came from!
+          redirect_back_or_default(root_path)
         end
       end
     end
+
+    private
+
+    # Updates the order's prices and all line items prices
+    def update_order!
+      @order = current_order
+      if update_line_items!
+        @order.update_attributes!(currency: @currency.iso_code)
+        @order.update!
+      end
+    end
+
+    # Updates prices of order's line items
+    def update_line_items!
+      return unless @order.line_items.any?
+      @order.line_items.each do |line_item|
+        update_line_item_price!(line_item)
+      end
+    end
+
+    # Returns the price object from given item
+    def price_from_line_item(line_item)
+      return unless line_item.variant
+      line_item.variant.prices.detect { |p| p.currency == @currency.iso_code }
+    end
+
+    # Updates price from given line item
+    def update_line_item_price!(line_item)
+      price = price_from_line_item(line_item)
+      if price
+        # Mass Assignment Protection \o/
+        line_item.currency = price.currency
+        line_item.price = price.amount
+        line_item.save!
+      end
+    end
   end
 end

spec/features/order_prices_spec.rb

@@ -0,0 +1,25 @@
+require 'spec_helper'
+
+describe 'Order' do
+  let!(:product) {create(:product)}
+
+  before do
+    reset_spree_preferences do |config|
+      config.supported_currencies = "USD,EUR,GBP"
+      config.allow_currency_change = true
+      config.show_currency_selector = true
+    end
+    create(:price, variant: product.master, currency: 'EUR', amount: 16.00)
+    create(:price, variant: product.master, currency: 'GBP', amount: 23.00)
+  end
+
+  context 'when existing in the cart', :js => true do
+    it "changes its currency, if user switches the currency." do
+      visit spree.product_path(product)
+      click_button 'Add To Cart'
+      expect(page).to have_content("$19.99")
+      select "EUR", :from => "currency"
+      expect(page).to have_content("€16.00")
+    end
+  end
+end