How can I add all child simple products to a Magento collection?

Question

I'm getting a collection of products which are on a customer's wishlist by passing in the IDs of those products (a mix of simple and configurable products) in the $customerListItemIds array and then running a stock check on them in the foreach loop. This is working fine except it's flagging up all configurable products as having 0 stock when, actually, their child products do have stock.

So I tried adding the visibility and type_id filters to the collection to skip configurable products and to include all simple products but this isn't working – the child simple products are not being added to the collection.

My question is how can I include the children of the configurable products and not include their parent?

<?php

    $lowStockCollection = array();
    $stockThreshold = Mage::getStoreConfig('cataloginventory/options/stock_threshold_qty');

    ...

    $productCollection = Mage::getModel('catalog/product')
                         ->getCollection()
                         ->addAttributeToSelect('*')
                         ->joinField('qty',
                             'cataloginventory/stock_item',
                             'qty',
                             'product_id=entity_id',
                             '{{table}}.stock_id=1',
                             'left')
                         ->addAttributeToFilter('visibility', array(1,2,3,4))
                         ->addAttributeToFilter('type_id', Mage_Catalog_Model_Product_Type::TYPE_SIMPLE)
                         ->addAttributeToFilter('qty', array('gt' => 0))
                         ->addAttributeToFilter('entity_id', array('in' => $customerListItemIds));

?>

<?php foreach($productCollection as $product): ?>
        <?php if($product->getQty() < $stockThreshold): ?>
            <?php $lowStockCollection[] = $product; ?>
        <?php endif; ?>
<?php endforeach; ?>

Answer 1

The problem is here:

->addAttributeToFilter('entity_id', array('in' => $customerListItemIds));

By adding this, you are allowing products that are also in the $customerListItemIds array. So if you set a configurable product ID in here then it's going to allow it to be added regardless of your other conditions. You should add a routine that takes any parent ID and grab the children prior to passing it to the collection.

Keep that addAttributeToFilter line in but add this above your $productCollection:

foreach ($customerListItemIds as $customer_item):
    $product=Mage::getModel('catalog/product')->load($customer_item);
    if ($product->getTypeId()=="configurable"):
        $child_ids = Mage::getModel('catalog/product_type_configurable')->getChildrenIds($product->getId());
        foreach ($child_ids as $cid):
            $customerListItemIds[]=$cid; // Adds the child products to the $customerListItemIds if it's a configurable product.
        endforeach;
    endif;
endforeach;