Magento Collection Catch-22 - magento

Question

I don't really understand how Magento collections work, so hopefully this is a simple question...
As the old adage goes, you can't observe an experiment without somehow altering it. This seems to hold true for Magento collections. I have a Featured Products module that I've written that works quite well. We have recently added Customer Reviews to our store. When viewing a category page it shows a random review of products in that category. This also works great. I added the review block to my Featured Products page, which was easy to do, but since those products aren't in a specific category, it just pulls a random, usually unrelated, review. To fix this, I modified my getProductCollection function in my Featured module and added the following to the end, after the collection has been created/saved:
$_product = $this->_productCollection->getFirstItem();
$_catIDs = $_product->getCategoryIds();
if(count($_catIDs) > 0)
{
$_cat = Mage::getModel('catalog/category')->load($_catIDs[0]);
Mage::register('current_category', $_cat);
}
Unfortunately, the mere act of looking at the first item in the collection breaks the pager in the toolbar. No matter which of the paging options I choose, it always shows all items when the above code is in place. If I comment out that section it works fine.
So my question is this: How can I get any information about the products in a collection without somehow changing the collection or breaking the paging?
Adding my code to help explain the problem more:
class VPS_Featured_Block_List extends Amasty_Sorting_Block_Catalog_Product_List//Mage_Catalog_Block_Product_List
{
protected function _getProductCollection()
{
if (is_null($this->_productCollection))
{
$_attributeNames = 'featured';
if($this->getAttributeName() != '')
$_attributeNames = $this->getAttributeName();
$_attrArray = explode(',', $_attributeNames);
$this->_productCollection = Mage::getModel('catalog/product')->getCollection();
$this->_productCollection->addAttributeToSelect('*');
$_filters = array();
foreach($_attrArray as $_attr)
$_filters[] = array('attribute' => $_attr, 'eq' => true);
$this->_productCollection->addFieldToFilter($_filters);
//$this->_productCollection->addFieldToFilter(array(array('attribute' => $_attr, 'eq' => true),));
Mage::getSingleton('cataloginventory/stock')->addInStockFilterToCollection($this->_productCollection);
Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($this->_productCollection);
Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($this->_productCollection);
//Get category of first product in collection (used for featured review)
$_catIDs = array();
$_product = $this->_productCollection->getFirstItem();
$_catIDs = $_product->getCategoryIds();
if(count($_catIDs) > 0)
{
$_cat = Mage::getModel('catalog/category')->load($_catIDs[0]);
Mage::register('current_category', $_cat);
}
}
return $this->_productCollection;
}
}

Answer 1

Thanks to #clockworkgeek for this answer he posted in my followup question:
The reason is because when you call getFirstItem() (or just about any other retrieval method) on a collection that collection is loaded. Any subsequent operation ignores the database and uses only the loaded data, filters have no effect because they are SQL only, ditto for pagination and selected columns. The workaround is to use a second collection based on the first.
$secondCollection = clone $firstCollection;
$secondCollection->clear();
$_foo123 = $secondCollection->getFirstItem();
The clear() method unloads the data for that collection, forcing it to access the database again next time

Answer 2

Hard to say what's going on without more context (magento version, which class you're in, etc.) BUT, what I think (60% confidence) it happening is you're getting a reference to the product collection before its filters have been added. Magento collections lazy load, which means database queries aren't run until you explicitly call load or you attempt to access an item. My guess (again, a guess) is that when you access the item above, the collection loads (with no filter). Then, other parts of the system add the filter but they're ignored because the collection is already loaded.
Answering your larger question is impossible without more context.