How to save advance inventory custom field value in db in Magento 2

Question

How to save advance inventory custom field value in DB on Magento 2

events.xml

<?xml version="1.0"?>
 <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
  <event name="cataloginventory_stock_item_save_after">
    <observer name="Vendor_Module::save_custom_data" 
         instance="Cm\Preorder\Observer\AfterProductSave"/>
  </event>
</config>

AfterProductSave.php

<?php
namespace Cm\Preorder\Observer;
use Magento\Framework\Event\ObserverInterface;
class AfterProductSave implements ObserverInterface
{
public function __construct(
    \Magento\Framework\App\RequestInterface $request
)
{
    $this->_request = $request;
}
 public function execute(\Magento\Framework\Event\Observer $observer)

 {
$post = $this->_request->getPost();
var_dump($post);
 }

}

Answer 1

created Observer which calls Cataloginventory Stock Item Save After event.

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="cataloginventory_stock_item_save_after">
        <observer name="Vendor_Module::save_custom_data" instance="Vendor\Module\Observer\AfterProductSave"/>
    </event>
</config>

And In Your AfterProductSave.php file You Can Add Your Custom Field Value

Answer 2

In this case, you need to implement extension attribute for Magento\CatalogInventory\Api\Data\StockItemInterface.

Use these two fields as extension_attributes on Magento\CatalogInventory\Api\Data\StockItemInterface

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for=Magento\CatalogInventory\Api\Data\StockItemInterface">
        <attribute code="preorder_button" type="string" />
        <attribute code="preorder_note" type="string" />
    </extension_attributes>
</config>

Save on using Plugin

Then create plugin on Magento\CatalogInventory\Api\StockItemRepositoryInterface:save

and this attribute value.

Like below

<?php
namespace {YourNamespace};

class SaveDataPlugin {

    public function beforeSave
    (
        \Magento\CatalogInventory\Api\StockItemRepositoryInterface $subject,
        \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem
    ) {
        $extensionAttributes = $subject->getExtensionAttributes();
        if (
                null !== $extensionAttributes &&
                null !== $extensionAttributes->getPreorderButton()
        ) {
            $Value = $extensionAttributes->getPreorderButton()->getValue();
            $stockItem->setPreorderButton($Value);
        }
        if (null !== $extensionAttributes &&
            null !== $extensionAttributes->getPreorderNote()
        ) {
            $Value = $extensionAttributes->getPreorderNote()->getValue();
            $stockItem->setPreorderNote($Value);
        }        
        return $stockItem;
    }

}

Note that code is posted base on Idea, not Test