Magento产品网格。 显示可配置的产品数量作为关联的简单产品的总和

I'm attempting to get Magento to show a Configurable product's stock base in the product grid. Obviously Configurable products don't have a quantity, so I mean I want to modify the way that product quantity is shown by some logic similar to:

if (product != configurable) 
{qty = do what it normally does}

else 
{get collection of associated products.
qty = sum of qty of that collection;}

I've found several controllers with reference to quantity such as this Block/Catalog/Product/grid.php

protected function _prepareCollection()
{
    $collection = $this->getCollection();
    if($queryString = $this->getQueryStr()) {
        $collection = Mage::Helper('enhancedgrid')
            ->getSearchCollection($queryString, $this->getRequest());
    }
    if(!$collection) {
        $collection = Mage::getModel('catalog/product')->getCollection();
    }
    $store = $this->_getStore();
    $collection 
        ->joinField('qty',
            'cataloginventory/stock_item',
            'qty',
            'product_id=entity_id',
            '{{table}}.stock_id=1',
            'left');
    $collection->addAttributeToSelect('sku');

But I haven't been able to get anywhere on it.

If none of this is possibly - just adding the 'Stock Availability' result would suffice.

Edit: ^This won't work as the configurable products are "IN STOCK" but all associated products are "OUT OF STOCK"

Thanks