Cannot sorting joined column in Magento Grid

[Do Quang Hung]

I have joined 2 columns to customer grid, I can make them filterable but I don't know how to make them sortable. Here is my _prepareCollection function:

protected function _prepareCollection()
  {
    $collection = Mage::getResourceModel('customer/customer_collection')
      ->addNameToSelect()
      ->addAttributeToSelect('email')
      ->addAttributeToSelect('created_at')
      ->addAttributeToSelect('group_id');
    $collection->addSalesInfoPerCustomer();
    $this->setCollection($collection);
    return parent::_prepareCollection();
  }

My joined function:

public function addSalesInfoPerCustomer() {
    $this->getSelect()
      ->joinLeft(
        array('s' => new Zend_Db_Expr('(SELECT sum(o.base_grand_total) total_sales, COUNT(*) as order_count, customer_id'
          . ' FROM ' . Mage::getSingleton('core/resource')->getTableName('sales/order') . ' AS o'
          . ' GROUP BY customer_id)')),
        "s.customer_id = e.entity_id",
        array('total_sales', 'order_count')
      );
  }
Added Column:
$this->addColumn('order_count', array(
      'header' => Mage::helper('customer')->__('Total Orders'),
      'index' => 'order_count',
      'filter_index' => 'pages_with_number_of_views.order_count',
      'filter_condition_callback' => array($this, '_roleFilter'),
    ));

    $this->addColumn('total_sales', array(
      'header' => Mage::helper('sales')->__('Total Sales'),
      'index' => 'total_sales',
      'type' => 'price',
      'currency_code' => Mage::app()->getStore()->getBaseCurrency()->getCode(),
      'filter_condition_callback' => array($this, '_roleFilterTotalSales'),
    ));