productorder.php

<?php

/**
 * Does a product order.
 *
 * @param  string $name
 * @param  int $quantity
 * @return bool
 */
public function doProductOrder($name, $quantity)
{
        $entry = $this->_getMatchingEntryByName($name);

        if ($entry)
        {
                $oldStock = $entry->hardwareStock;
                $newStock = $this->_reduceEntryInventory($entry, $quantity);

                return $oldStock != $newStock;
        }

        Craft::log('No entry found for this product name.', LogLevel::Error);

        return false;
}

/**
 * Reduce the inventory value for an entry, by the supplied quantity
 *
 * @param  EntryModel $entry    a Craft entry
 * @param  int        $quantity the value to subtract from the current inventory
 * @return int                  the new adjusted inventory value
 */
private function _reduceEntryInventory($entry, $quantity)
{
        $currentStock = $entry->hardwareStock;

        if ($currentStock >= $quantity)
        {
                $currentStock = $currentStock - $quantity;

                $entry->setContentFromPost(array(
                    'fieldToChange' => $currentStock
                ));

                // Let's see if it saves.
                // Field validation should happen automatically
                if (craft()->entries->saveEntry($entry))
                {
                        Craft::log('Entry updated (ID: '.$entry->id.').');
                }
                else
                {
                        Craft::log('Could not updated the entry (ID: '.$entry->id.').', LogLevel::Error);
                }
        }

        return $currentStock;
}

/**
 * Fetches an entry model by its product name.
 *
 * @param  string $name
 * @return null|EntryModel
 */
private function _getMatchingEntryByName($name)
{
        $criteria = craft()->elements->getCriteria(ElementType::Entry);
        $criteria->productName = $name;

        return $criteria->first();
}