Magento – Add Filter to magento 2 category product collection

Looking at the files which are relevant for this, I can identify Varien_Data_Collection which is the base class and implements IteratorAggregate, so you can loop over collections using foreach($_productCollection as $product).

It does so via the method getIterator() which mainly returns new ArrayIterator($this->_items);

In contrast, a level further up extending from Varien_Data_Collection there is the class Varien_Data_Collection_Db.

This class implements the method getData() as follows:

public function getData()
{
    if ($this->_data === null) {
        $this->_renderFilters()
             ->_renderOrders()
             ->_renderLimit();
        $this->_data = $this->_fetchAll($this->_select);
        $this->_afterLoadData();
    }
    return $this->_data;
}

It explicitly uses the _renderOrders() method which adds an order clause to the select statement. This is why getData() will use your given order, but the default iteration will not.

Price filter uses a bit different SQL query. It tries to avoid joining with the product entity table and replace it with the price index.

As you are using layered navigation module, you need to modify class app\code\local\Amasty\Shopby\Model\Mysql4\Price.php instead of the magento default.