Build list of variants removed from the cart when overwriting

app/models/spree/order_populator_decorator.rb

@@ -75,6 +75,12 @@ Spree::OrderPopulator.class_eval do
     li_added || li_quantity_changed || li_max_quantity_changed
   end
 
+  def variants_removed(variants_data)
+    variant_ids_given = variants_data.map { |data| data[:variant_id] }
+
+    (variant_ids_in_cart - variant_ids_given).uniq
+  end
+
   def check_order_cycle_provided_for(variant)
     order_cycle_provided = (!order_cycle_required_for(variant) || @order_cycle.present?)
     errors.add(:base, "Please choose an order cycle for this order.") unless order_cycle_provided
@@ -97,4 +103,8 @@ Spree::OrderPopulator.class_eval do
   def line_item_for_variant_id(variant_id)
     order.find_line_item_by_variant Spree::Variant.find(variant_id)
   end
+
+  def variant_ids_in_cart
+    @order.line_items.map &:variant_id
+  end
 end

spec/models/spree/order_populator_spec.rb

@@ -46,7 +46,6 @@ module Spree
     end
 
     describe "varies_from_cart" do
-      #let(:order) { create(:order) }
       let(:variant) { double(:variant, id: 123) }
 
       it "returns true when item is not in cart and a quantity is specified" do
@@ -93,6 +92,28 @@ module Spree
       end
     end
 
+    describe "variants_removed" do
+      it "returns the variant ids when one is in the cart but not in those given" do
+        op.stub(:variant_ids_in_cart) { [123] }
+        op.send(:variants_removed, []).should == [123]
+      end
+
+      it "returns nothing when all items in the cart are provided" do
+        op.stub(:variant_ids_in_cart) { [123] }
+        op.send(:variants_removed, [{variant_id: 123}]).should == []
+      end
+
+      it "returns nothing when items are added to cart" do
+        op.stub(:variant_ids_in_cart) { [123] }
+        op.send(:variants_removed, [{variant_id: 123}, {variant_id: 456}]).should == []
+      end
+
+      it "does not return duplicates" do
+        op.stub(:variant_ids_in_cart) { [123, 123] }
+        op.send(:variants_removed, []).should == [123]
+      end
+    end
+
     describe "attempt_cart_add" do
       it "performs additional validations" do
         variant = double(:variant)