Magento collection issues, methods not working filters not applied etc

Question

Trying to create a couple of quick extensions, one to display a list of best sellers and one to display a list of items on promotion. I want both of these modules to filter by a given cateogry id if one is supplied. So I found out that magento has a built in module that isn't used in the default theme namely the catalog/product_list_promotion rather than create something from scratch I thought I would try to modify this module and also use it as a base for the best seller module. So I found some examples of a best seller module and used the code to create best seller module, both the promotion module and the best seller module are working and displaying products. The problem is I can't apply category filters and I can't limit the number of results, I've tried every combination of adding the filters/limits/page sizes etc and either i get no errors but the original unchanged output or a fatal error to an undefined method. I have searched for hours for a solution and found some similar questions on SO and other forums, all suggest what I have already tried which hasn't worked, maybe someone can help?

So in Mage/Catalog/Product/Block/List/promotion.php I have:

class Mage_Catalog_Block_Product_List_Promotion extends Mage_Catalog_Block_Product_List
{
    protected function _getProductCollection()
    {
        if (is_null($this->_productCollection)) {
            $collection = Mage::getResourceModel('catalog/product_collection');
            Mage::getModel('catalog/layer')->prepareProductCollection($collection);
            // your custom filter
        $collection->getSelect()->order('rand()');
            $collection->addAttributeToFilter('promotion', 1)->addStoreFilter();
            $this->_productCollection = $collection;
        }
        return $this->_productCollection;
    }
}

I've tried adding $c...->setLimit(X), $c...->limit(X) $c...->getSelect()->limit(X), $c...->page(1, X), $c...->setPageSize(X) both before and after prepareCollection... In most cases I get no errors but no limit is applied. the best sellers module is essentially the same so wont post the code as I assume the issue with this will be the same for the best sellers module.

Any help would be greatly appreciated.

Answer 1

Tried adding $collection->load() and this seems to fix the problem, I assume my filters etc were being overridden in another class somewhere when the $collection is loaded. I didn't do this initially I was under the impression Magento didn't load the collection until it was needed allowing you to modify it at any point until it is loaded, I wanted to keep that functionality...