Removing a product from the collection on a category

Question

I have a Magento site where I need to remove some products from certain customer groups due to them having a zero price on the products.

I have tried catalog_product_collection_load_after event and have successfully managed to remove the products that I want. This however causes a gap on the catagory pages where the products would be. Would there be any way to go about remove products from a collection either before the pagination is set on the collection or to "remake" the collection once you've removed the products.

The code I've tried so far:

 /* Remove all products from the category list where the (group) price is zero */
public function removeZeroPriceProducts($observer) {

    if (! Mage::app()->getStore()->isAdmin())
    {
        $collection = $observer->getEvent()->getCollection();
        //Loop through all products in the collection
        foreach ($collection as $product)
        {

            $origProduct = $product;

            //If it is a configurable we need to get the childrens prices.
            // Since if the customer isn't allowed to see the product all prices on all children will be zero we only need to get any of the children
            if($this->getTypeId() == "configurable") {
                $product = $product->getTypeInstance()->getUsedProductCollection()->addAttributeToSelect('price')->addPriceData()->setPageSize(1)->getFirstItem();
            }

            //Check if the price is zero. If so remove the product from the collection. Note that this --will-- cause a gap in the end of the page before the pagination.
            if($origProduct->getFinalPrice() === 0 || $product->getFinalPrice() === 0)
                $observer->getEvent()->getCollection()->removeItemByKey($origProduct->getId());
        }
    }
}

Edit: I've tried added it to _getProductCollection in List.php as suggested by Mohit. This removes the product as expected but unfortunately still leaves a gap. I'll leave the code for that here as well.

protected function _getProductCollection()
{
    if (is_null($this->_productCollection)) {
        $layer = $this->getLayer();
        /* @var $layer Mage_Catalog_Model_Layer */
        if ($this->getShowRootCategory()) {
            $this->setCategoryId(Mage::app()->getStore()->getRootCategoryId());
        }

        // if this is a product view page
        if (Mage::registry('product')) {
            // get collection of categories this product is associated with
            $categories = Mage::registry('product')->getCategoryCollection()
                ->setPage(1, 1)
                ->load();
            // if the product is associated with any category
            if ($categories->count()) {
                // show products from this category
                $this->setCategoryId(current($categories->getIterator()));
            }
        }

        $origCategory = null;
        if ($this->getCategoryId()) {
            $category = Mage::getModel('catalog/category')->load($this->getCategoryId());
            if ($category->getId()) {
                $origCategory = $layer->getCurrentCategory();
                $layer->setCurrentCategory($category);
            }
        }
        $this->_productCollection = $layer->getProductCollection();

        $this->prepareSortableFieldsByCategory($layer->getCurrentCategory());

        if ($origCategory) {
            $layer->setCurrentCategory($origCategory);
        }
    }

    //Try and remove a specific ID from the collection
    $collection = $this->_productCollection;
    $collection->removeItemByKey(4146);
    return $collection;
}

Answer 1

To modify product collection before rendering category page follow these steps:

1. Override Magento\Catalog\Block\Product\ListProduct
2. In function _getProductCollection after if statement insert your code:

$collection = $this->_productCollection;
foreach ($collection as $k => $product){
 // Here goes your code to modify product
 // For example:

$collection->removeItemByKey($k);// Remove original item from collection 
}
$collection->setOrder('position','ASC');// Reorder collection as you want
return $collection;

Answer 2

I found a solution that seems to work. On the customer_login event I loop through all the products and add all the products that the customer are not allowed to see into an array that I then save in a customer session variable.

I then create a function called removeZeroPriceProducts that catches onto the catalog_product_collection_load_before event. In this function I've excluded the products by doing the following:

/* Remove all products from the category list where the (group) price is zero */
public function removeZeroPriceProducts($observer) {

    if (! Mage::app()->getStore()->isAdmin())
    {
        //Get all disallowed products for the user
        if($disallowed = Mage::getSingleton('checkout/session')->getDisallowedProducts()){
            //Add a filter for the product collection that removes all the disallowed products
            $observer->getEvent()->getCollection()->addFieldToFilter('entity_id', array('nin' => $disallowed));
        }
    }
}

Instead of using removeItemByKey which removed the product but left a gap I now instead add a filter which removes all products that can be found in my session variable.

Note. I first tried to loop through all the products in the removeZeroPriceProducts fonction. This however broke the page which is why I instead added all the products to a session variable when the customer logs in