Backorders not working when product is marked as "Out of Stock" (Can't add product to cart)

[nikoelgatito]

From the official Magento documentation both for the first edition and Magento 2 we can read the following very logical definition for Out of Stock products:

Out of Stock: Unless Backorders are activated, prevents the product from being available for purchase and removes the listing from the catalog.

Source: http://docs.magento.com/m2/ce/user_guide/catalog/settings-advanced-advanced-inventory.html

Preconditions

1. Magento 2.3.x
2. Edit/Create a product,
3. Set Manage Stock to "Yes",
4. Set Qty to "0",
5. Enable Backorders
6. Set Stock Status to "Out of Stock".

Steps to reproduce

1. Visit the product page,
2. Add the product to cart.

Expected result

1. The Add to Cart button shows and we can add the product to the shopping cart.

Actual result

1. The Add to Cart button does not show,
2. Even after forcing the isSalable() method to return true, then refreshing the page and clicking on the Add to Cart button that is now showing, the product is not added to cart and the red "This product is out of stock." notification message is showing.

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Remarks regarding the Add to Cart button not showing:

It seems that the isSalable() method depends more on the In Stock status than the real item quantity. This function should rely on real quantity for every product that has the Manage Stock option enabled. Solely relying on the Stock Status for product with real quantity is not reliable.

The logic should be as follow:
Manage Stock = No:

• isSalable() should return true/false based on Stock Status.

Manage Stock = Yes:

1. Backorders are not allowed: isSalable() should return true/false based on stock qty and threshold.
2. Backorders are allowed: isSalable() should always return true.

https://stackoverflow.com/questions/9038988/what-is-the-difference-between-issaleable-and-isavailable

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Remarks regarding the product not adding to cart:

It seems that the "magic" preventing the product to be added to cart reside in this class:
Magento\CatalogInventory\Model\Quote\Item\QuantityValidator::validate()

As we can see on line 103, the verification here again solely rely on Stock Status and not on the actual quantity even though Manage Stock is enabled:

103 | if (!$stockItem->getIsInStock() || $parentStockItem && !$parentStockItem->getIsInStock()) {
       ...
      }

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This is unfortunate as we though backorders were well implemented in Magento and we are currently on a project for which we will have to modify the default (wrong) Magento behavior by the meantime this bug gets officially resolved and released.

[UPDATE]

Possible Solution:

It seems that these two small changes would correct this bug!

Magento\CatalogInventory\Model\Quote\Item\QuantityValidator::validate()

Line 103 | - if (!$stockItem->getIsInStock() || $parentStockItem && !$parentStockItem->getIsInStock()) {
Line 103 | + if ((!$stockItem->getIsInStock() || $parentStockItem && !$parentStockItem->getIsInStock()) && !$stockItem->getBackorders()) {

Magento\CatalogInventory\Model\StockStateProvider::checkQuoteItemQty()

Line 155 | - if (!$stockItem->getIsInStock()) {
Line 155 | + if (!$stockItem->getIsInStock() && !$stockItem->getBackorders()) {

These are making sure that the product Stock Status being out of stock has no impact if backorders are enabled. The process will then continue its nomal verification flow and the backorder notices will still be shown where applicable. It has been tested and completely working.

[smoskaluk]

@nikoelgatito Unfortunally, despite what is written in docs.magento, products with status “Out of stock” couldn’t be available for purchase. If you turned on “Backorder” function, it makes your product “In stock” even with negative qty in stock, so it remains purchasable

[nikoelgatito]

Thank you @smoskaluk for your comments,

I do understand how it is currently unlogically working. I also do understand that basically the Stock Status attribute was to control the availability of an item for which stocks are not managed. However when managing stocks this logic does not apply. An item for which a store accept backorders should always be recognized as not in stock as it is not, stocks might be on their way already or will be once an order has been placed (special order) but in any cases it should be recognized as out of stock. Most store managing stocks and backorders will want the threshold to switch Stock Status to Out of Stock for all products that has a qty below what they consider unavailable. Still they would like to be able to use the backorder function. The issue here is most merchant think that way because it is simply logically the way to do it but as Magento did it wrong unfortunately everyone had to adapt. What the doc says is the right way to do it and the very simple solution that is perfectly working for us is the fixed I shared above.

After all the Stock Status attribute could almost be nonexistent and the isSalable() function could simply rely on real stocks for every products that has its stocks managed. Some might say that the Stock Status attribute acts more as a "Force Availability" attribute than anything but this does not add up to the current Magento logic. The backorders attribute should be switched if one is willing to allow or disallow negative qty. In my point of view, Stock Status is there more as informative to the visitor than anything, it is simply an indicator, whatever the current qty is, that the merchant decides from this point on this product should be indicated as out of stock. Still if backorders are enabled it should be saleable.

Say for example that a store has an unreliable inventory, an inventory that is updating every hour with lots of stock movements. They do accept backorders but the processing delay raise up to three days instead of 1 day for in stock products. Then to better be safe than sorry, they decide that the out of stock threshold is 5 qty. This way they ensure that a customer has much less chance to perceive unforeseen extra delays. Now what happens, the qty goes to 5 qty and the product is marked as out of stock, this makes sure that all visitors are aware that this is currently out of stock (backordered) and that an additional delay is to be expected. Unfortunately, this very popular situation does not work with the current Magento implemented logic, the product won't be saleable! The only way to make it saleable would be to put it In Stock, "lying" to ourselves and to the visitors showing it is in stock and "hopefully" we can add a notice in the cart.

Now of course added to the fix I proposed above, a great idea would be a way to "override" the Out of Stock status showing on the product page for Backordered if backorders are enabled for this product, this would be even more transparent to the customer. This is a part of what we do with a custom extension.

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At this time, my understanding is that there probably was two different developers working on this aspect of the stock management for Magento and both did go with a different approach reaching in the middle.

This unfortunately does not work as there is a large difference between:

1. A displayed stock status that serves as indicator to the customer.
2. The actual availability or saleability of the product.
3. The real physical qty available.

These surely should be related and are meant to influence each other but they should still be independant and a product should definitely still be saleable while showing out of stock or backorder on the frontend. A merchant should also be able to control from which qty level a product will be displayed as out of stock.

The solution # 2 below should allow for all case scenario.

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Solution # 2 (Best)

The solution I initially proposed when creating this bug report is the simplest way requiring only a few edits for achieving what most users expect from a stock management logic. There is of course a much better approach to this that would require more code change but would also allow for better understanding, flexibility and much more satisfied merchants and customers.

The much better way to make it work keeping the actual Stock Status "almost" how it is (directly control the product saleability) would be in the case where ALL the following would apply:
1. The current Stock Status attribute would be called "Availability" [Allow Sale/Disallow Sale] which would not relate to actual physical stocks BUT to the fact that we allow of disallow this product to be ordered. In this case it would be a "simple" way to keep a product enabled but still prevent sales.
2. The stock status displayed for a product would be controlled by an isInStock() that would rely both on availability and qty versus threshold. This would NOT affect the saleability of the product itself, only the stock status to be displayed on the frontend. The function logic would be the following:

true - `manage stock` is set to No.
true - `manage stock` is set to Yes or qty is above threshold
false - isAvailable() returns false
false - `manage stock` is set to Yes and qty is under threshold

3. In the case where isInStock() returns false but isAvalaible() returns true, isSalable() should verify if backorders are enabled and return the result. This also means that backorders logic should NOT solely be relying on qty < 0 to be taken into account.

This would mean that a product could appear out of stock if its qty is below the threshold even though availability is set to "available" and it would still be possible to order if backorders are enabled.

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For those whose requirements include a way to disallow sales when a certain threshold has been reach (as with current wrong logic), my Solution # 2 still allows it:
Simply setting up an out of stock threshold as usual and making sure that backorders are disabled should be enough to ensure that both isInStock() and isSalable() return false therefore making the product both showing out of stock while preventing it sales once the threshold has been reached.