Orsdxy

I'm using Magento 1.7.0.2 and Magento 1.9.1.0.

As we know, by default, Qty Inventory at the backend (Admin - Catalog - Manage Products - Edit - General - Inventory - Qty) may contain a negative value, such as -1.

How to prevent the quantity of inventory in the backend products is negative?

You could create a custom module and an event observer hooked to the cataloginventory_stock_item_save_before event then you can validate if the qty will be saved as negative and perform some action.

Configuration:

Vendor/Module/etc/config.xml

<global>

...

    <events>

        <cataloginventory_stock_item_save_before>

            <observers>

                <vendor_module_stock_item_save_before>

                    <class>vemdor_module/observer</class>

                    <method>stockItemSaveBefore</method>

                </vendor_module_stock_item_save_before>

            </observers>

        </cataloginventory_stock_item_save_before>

    </events>

</global>

...

Observer:

Vendor/Module/Model/Observer.php

class Vendor_Module_Model_Observer

{

   public function stockItemSaveBefore(Varien_Event_Observer $observer)

   {

        $item = $observer->getEvent()->getItem();

        // Perform some action ...

   }

}

I don't know what do you want to do with that information. You may want to send an email, save a log or just keep the qty in 0, regardless the action you'll take this is the way to do it.

I have seen constellations where qty is negative / < zero in a very small amount of client systems that we did not create from the scratch. The problem is, that the products with negative qty are "in stock/available" and can be bought by shop customers.

Instead of wasting computation time i would add a trigger:

delimiter //

CREATE TRIGGER negative_stock_to_zero BEFORE UPDATE ON cataloginventory_stock_item 

FOR EACH ROW 

BEGIN

    IF NEW.qty < 0 THEN

       NEW.qty = 0;

    END IF;

END;//

delimiter;

Of course you could also check if backordering is enabled/disabled in the IF-Clause - but thats up to you.

In the admin console under System -> Configuration

Under Catalog -> Inventory

You should see a setting in the Product Stock Options called Backorders. Set this to No Backorders.

Additionally set the "Qty for Item's Status to Become Out of Stock" setting to 0.