Don't serialize all variant override objects where they are not needed

This is only needed in one place, and only as a count, and can cause huge amounts of superfluous data to be fetched and serialized, for example in the admin products index page (where variant override data is not used at all).

app/serializers/api/admin/variant_serializer.rb

@@ -5,11 +5,9 @@ module Api
     class VariantSerializer < ActiveModel::Serializer
       attributes :id, :name, :producer_name, :image, :sku, :import_date,
                  :options_text, :unit_value, :unit_description, :unit_to_display,
-                 :display_as, :display_name, :name_to_display,
+                 :display_as, :display_name, :name_to_display, :variant_overrides_count,
                  :price, :on_demand, :on_hand, :in_stock, :stock_location_id, :stock_location_name
 
-      has_many :variant_overrides
-
       def name
         if object.full_name.present?
           "#{object.name} - #{object.full_name}"
@@ -55,6 +53,10 @@ module Api
 
         object.stock_items.first.stock_location.name
       end
+
+      def variant_overrides_count
+        object.variant_overrides.count
+      end
     end
   end
 end

app/views/spree/admin/products/index/_products_variant.html.haml

@@ -29,6 +29,6 @@
   %td.actions
     %a{ 'ng-click' => 'editWarn(product,variant)', :class => "edit-variant icon-edit no-text", 'ng-show' => "variantSaved(variant)", 'ofn-with-tip' => t(:edit)  }
   %td.actions
-    %span.icon-warning-sign{ 'ng-if' => 'variant.variant_overrides', 'ofn-with-tip' => "{{ 'spree.admin.products.index.products_variant.variant_has_n_overrides' | t:{n: variant.variant_overrides.length} }}" }
+    %span.icon-warning-sign{ 'ng-if' => 'variant.variant_overrides_count > 0', 'ofn-with-tip' => "{{ 'spree.admin.products.index.products_variant.variant_has_n_overrides' | t:{n: variant.variant_overrides_count} }}" }
   %td.actions
     %a{ 'ng-click' => 'deleteVariant(product,variant)', "ng-class" => '{disabled: product.variants.length < 2}', :class => "delete-variant icon-trash no-text", 'ofn-with-tip' => t(:remove)  }