how to add negative qty product into cart in magento. 2

Question

How to add a out of stock (less than 0 ) product in shopping cart in magento 2

In my backorders setting i have custom option as Allow pre-order when i select a custom option and saved a backorder configuration after that how to add negative qty product into cart in magento.

Product edit page in admin

Store > configuration > catalog > inventory

I can't add a product into cart

Please some one help me to solve this

Answer 1

Try the following way:

app/code/SR/MagentoCommunity/etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\CatalogInventory\Model\StockStateProvider">
        <plugin name="sr_custom_backorders"
                type="SR\MagentoCommunity\Plugin\CatalogInventory\Model\StockStateProvider" sortOrder="1"/>
    </type>
</config>

Suppose you custom option Allow Pre-Order value 5.

app/code/SR/MagentoCommunity/Plugin/CatalogInventory/Model/StockStateProvider.php

<?php
namespace SR\MagentoCommunity\Plugin\CatalogInventory\Model;

use Magento\CatalogInventory\Api\Data\StockItemInterface;

class StockStateProvider
{
    /**
     * @param \Magento\CatalogInventory\Model\StockStateProvider $subject
     * @param $result
     * @param StockItemInterface $stockItem
     * @param $qty
     * @return bool
     */
    public function afterCheckQty(
        \Magento\CatalogInventory\Model\StockStateProvider $subject,
        $result,
        StockItemInterface $stockItem,
        $qty
    ) {
        if (!$result) {
            if ($stockItem->getQty() - $stockItem->getMinQty() - $qty < 0) {
                switch ($stockItem->getBackorders()) {
                    case \Magento\CatalogInventory\Model\Stock::BACKORDERS_YES_NONOTIFY:
                    case \Magento\CatalogInventory\Model\Stock::BACKORDERS_YES_NOTIFY:
                    case 5:
                        return true;
                    default:
                        return false;
                }
            }
        }

        return $result;
    }
}

[Update]

Using around plugin:

app/code/SR/MagentoCommunity/Plugin/CatalogInventory/Model/StockStateProvider.php

<?php
namespace SR\MagentoCommunity\Plugin\CatalogInventory\Model;

use Magento\CatalogInventory\Api\Data\StockItemInterface;

class StockStateProvider
{
    /**
     * @param \Magento\CatalogInventory\Model\StockStateProvider $subject
     * @param \Closure $proceed
     * @param StockItemInterface $stockItem
     * @param $qty
     * @return bool|mixed
     */
    public function aroundCheckQty(
        \Magento\CatalogInventory\Model\StockStateProvider $subject,
        \Closure $proceed,
        StockItemInterface $stockItem,
        $qty
    ) {
        $result = $proceed($stockItem, $qty);
        if (!$result) {
            if ($stockItem->getQty() - $stockItem->getMinQty() - $qty < 0) {
                switch ($stockItem->getBackorders()) {
                    case \Magento\CatalogInventory\Model\Stock::BACKORDERS_YES_NONOTIFY:
                    case \Magento\CatalogInventory\Model\Stock::BACKORDERS_YES_NOTIFY:
                    case 5:
                        return true;
                    default:
                        return false;
                }
            }
        }

        return $result;
    }
}

Answer 2

Stroes -> Configuration -> Catalog -> Inventory 

Click on Product stock Options Tab.

Answer 3

1 .Set System - Configuration - Catalog - Inventory - Stock Options - Display Out of Stock Products to Yes.

2.Set System - Configuration - Catalog - Inventory - Product Stock Options - Backorder to Allow Qty Below 0.