Bring availability validator from spree_core

app/models/spree/stock/availability_validator.rb

@@ -0,0 +1,25 @@
+module Spree
+  module Stock
+    class AvailabilityValidator < ActiveModel::Validator
+      def validate(line_item)
+        if shipment = line_item.target_shipment
+          units = shipment.inventory_units_for(line_item.variant)
+          return if units.count > line_item.quantity
+          quantity = line_item.quantity - units.count
+        else
+          quantity = line_item.quantity
+        end
+
+        quantifier = Stock::Quantifier.new(line_item.variant_id)
+
+        unless quantifier.can_supply? quantity
+          variant = line_item.variant
+          display_name = %Q{#{variant.name}}
+          display_name += %Q{ (#{variant.options_text})} unless variant.options_text.blank?
+
+          line_item.errors[:quantity] << Spree.t(:out_of_stock, :scope => :order_populator, :item => display_name.inspect)
+        end
+      end
+    end
+  end
+end