<?php
class Rayfox_Catalog_Model_Layer extends Mage_Catalog_Model_Layer
{
     public function prepareProductCollection($collection)
     {
          if (Mage::helper('catalog')->isModuleEnabled('Mage_CatalogInventory')) {
            Mage::getModel('cataloginventory/stock_item')->addCatalogInventoryToProductCollection($collection);
        }
       
        $collection->getSelect()->order('is_in_stock desc');
       
        //Mage::log($collection->getSelect() . '');
       
          return parent::prepareProductCollection($collection);
     }
}；