在product collision后面加入对产品是否有货的排序
//add code for product collection sort by stock
$productColl->getSelect()->joinLeft(
array('_inventory_table'=>$productColl->getTable('cataloginventory/stock_item')),
"_inventory_table.product_id = e.entity_id",
array('is_in_stock', 'manage_stock')
);
$productColl->addExpressionAttributeToSelect('on_top',
'(CASE WHEN (((_inventory_table.use_config_manage_stock = 1) AND (_inventory_table.is_in_stock = 1)) OR ((_inventory_table.use_config_manage_stock = 0) AND (1 - _inventory_table.manage_stock + _inventory_table.is_in_stock >= 1))) THEN 1 ELSE 0 END)',
array());
$productColl->getSelect()->order('on_top DESC');
