Magento 2.3.2: How to override a Inventory Deduction API model?

Question

I want to override below Magento core file

vendor/magento/module-inventory-source-deduction-api/Model/SourceDeductionService.php

to add code $qty=0; inline 83.

I tried by my self but getting an error.

Fatal error: Declaration of Ahmad\InventoryDeduction\Model\Rewrite\SourceDeductionService::execute(Ahmad\InventoryDeduction\Model\Rewrite\SourceDeductionRequestInterface $sourceDeductionRequest): void must be compatible with Magento\InventorySourceDeductionApi\Model\SourceDeductionServiceInterface::execute(Magento\InventorySourceDeductionApi\Model\SourceDeductionRequestInterface $sourceDeductionRequest): void in /opt/lampp/htdocs/M232/app/code/Ahmad/InventoryDeduction/Model/Rewrite/SourceDeductionService.php on line 8

Tried below module.

/Ahmad/Deduction/Model/Rewrite/SourceDeductionService.php

<?php
namespace Ahmad\Deduction\Model\Rewrite;
use Magento\Framework\Exception\LocalizedException;
use Magento\InventoryApi\Api\SourceItemsSaveInterface;
use Magento\InventoryConfigurationApi\Api\GetStockItemConfigurationInterface;
use Magento\InventorySalesApi\Api\GetStockBySalesChannelInterface;

class SourceDeductionService extends \Magento\InventorySourceDeductionApi\Model\SourceDeductionService
{
      /**
     * @var SourceItemsSaveInterface
     */
    private $sourceItemsSave;

    /**
     * @var GetSourceItemBySourceCodeAndSku
     */
    private $getSourceItemBySourceCodeAndSku;

    /**
     * @var GetStockItemConfigurationInterface
     */
    private $getStockItemConfiguration;

    /**
     * @var GetStockBySalesChannelInterface
     */
    private $getStockBySalesChannel;

    /**
     * @param SourceItemsSaveInterface $sourceItemsSave
     * @param GetSourceItemBySourceCodeAndSku $getSourceItemBySourceCodeAndSku
     * @param GetStockItemConfigurationInterface $getStockItemConfiguration
     * @param GetStockBySalesChannelInterface $getStockBySalesChannel
     */
    public function __construct(
        SourceItemsSaveInterface $sourceItemsSave,
        GetSourceItemBySourceCodeAndSku $getSourceItemBySourceCodeAndSku,
        GetStockItemConfigurationInterface $getStockItemConfiguration,
        GetStockBySalesChannelInterface $getStockBySalesChannel
    ) {
        $this->sourceItemsSave = $sourceItemsSave;
        $this->getSourceItemBySourceCodeAndSku = $getSourceItemBySourceCodeAndSku;
        $this->getStockItemConfiguration = $getStockItemConfiguration;
        $this->getStockBySalesChannel = $getStockBySalesChannel;
    }

    /**
     * @inheritdoc
     */
    public function execute(Ahmad\Deduction\Model\Rewrite\SourceDeductionRequestInterface $sourceDeductionRequest): void
    {
        $sourceItems = [];
        $sourceCode = $sourceDeductionRequest->getSourceCode();
        $salesChannel = $sourceDeductionRequest->getSalesChannel();

        $stockId = $this->getStockBySalesChannel->execute($salesChannel)->getStockId();
        foreach ($sourceDeductionRequest->getItems() as $item) {
            $itemSku = $item->getSku();
            $qty = $item->getQty();
            $stockItemConfiguration = $this->getStockItemConfiguration->execute(
                $itemSku,
                $stockId
            );

            if (!$stockItemConfiguration->isManageStock()) {
                //We don't need to Manage Stock
                continue;
            }

            $sourceItem = $this->getSourceItemBySourceCodeAndSku->execute($sourceCode, $itemSku);
            if (($sourceItem->getQuantity() - $qty) >= 0) {
                $qty = 0;   // To stop stock qty decrease twice, on invoice
                $sourceItem->setQuantity($sourceItem->getQuantity() - $qty);
                $sourceItems[] = $sourceItem;
            } else {
                throw new LocalizedException(
                    __('Not all of your products are available in the requested quantity.')
                );
            }
        }

        if (!empty($sourceItems)) {
            $this->sourceItemsSave->execute($sourceItems);
        }
    }
}

Ahmad/Deduction/etc/adminhtml/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">

    <preference for="Magento\InventorySourceDeductionApi\Model\SourceDeductionService" type="Ahmad\Deduction\Model\Rewrite\SourceDeductionService" />

</config>

Answer 1

There are many mistakes in your class.

1. You must include full path of the class for GetSourceItemBySourceCodeAndSku
2. You should call the parent __construct method.
3. Copy the exact argument list from the parent class for execute() method.

<?php
namespace Ahmad\Deduction\Model\Rewrite;
use Magento\InventoryApi\Api\SourceItemsSaveInterface;
use Magento\InventoryConfigurationApi\Api\GetStockItemConfigurationInterface;
use Magento\InventorySalesApi\Api\GetStockBySalesChannelInterface;
use Magento\InventorySourceDeductionApi\Model\GetSourceItemBySourceCodeAndSku;
use Magento\InventorySourceDeductionApi\Model\SourceDeductionRequestInterface;
use Magento\Framework\Exception\LocalizedException;
//use Magento\InventorySourceDeductionApi\Model\SourceDeductionService ;// added
/**
 * @inheritdoc
 */
class SourceDeductionService extends \Magento\InventorySourceDeductionApi\Model\SourceDeductionService
{

    /**
     * @var SourceItemsSaveInterface
     */
    private $sourceItemsSave;

    /**
     * @var GetSourceItemBySourceCodeAndSku
     */
    private $getSourceItemBySourceCodeAndSku;

    /**
     * @var GetStockItemConfigurationInterface
     */
    private $getStockItemConfiguration;

    /**
     * @var GetStockBySalesChannelInterface
     */
    private $getStockBySalesChannel;

    /**
     * @param SourceItemsSaveInterface $sourceItemsSave
     * @param GetSourceItemBySourceCodeAndSku $getSourceItemBySourceCodeAndSku
     * @param GetStockItemConfigurationInterface $getStockItemConfiguration
     * @param GetStockBySalesChannelInterface $getStockBySalesChannel
     */
    public function __construct(
        SourceItemsSaveInterface $sourceItemsSave,
        GetSourceItemBySourceCodeAndSku $getSourceItemBySourceCodeAndSku,
        GetStockItemConfigurationInterface $getStockItemConfiguration,
        GetStockBySalesChannelInterface $getStockBySalesChannel
    ) {
        $this->sourceItemsSave = $sourceItemsSave;
        $this->getSourceItemBySourceCodeAndSku = $getSourceItemBySourceCodeAndSku;
        $this->getStockItemConfiguration = $getStockItemConfiguration;
        $this->getStockBySalesChannel = $getStockBySalesChannel;
        parent::__construct($sourceItemsSave,$getSourceItemBySourceCodeAndSku,$getStockItemConfiguration,$getStockBySalesChannel); //ended the statement with a ;
    }

    /**
     * @inheritdoc
     */
    public function execute(SourceDeductionRequestInterface $sourceDeductionRequest): void
    {
        $sourceItems = [];
        $sourceCode = $sourceDeductionRequest->getSourceCode();
        $salesChannel = $sourceDeductionRequest->getSalesChannel();

        $stockId = $this->getStockBySalesChannel->execute($salesChannel)->getStockId();
        foreach ($sourceDeductionRequest->getItems() as $item) {
            $itemSku = $item->getSku();
            $qty = $item->getQty();
            $stockItemConfiguration = $this->getStockItemConfiguration->execute(
                $itemSku,
                $stockId
            );

            if (!$stockItemConfiguration->isManageStock()) {
                //We don't need to Manage Stock
                continue;
            }

            $sourceItem = $this->getSourceItemBySourceCodeAndSku->execute($sourceCode, $itemSku);
            if (($sourceItem->getQuantity() - $qty) >= 0) {
                $qty = 0;   //added   to stop decrease twice stock  qty  on invoice
                $sourceItem->setQuantity($sourceItem->getQuantity() - $qty);
                $sourceItems[] = $sourceItem;
            } else {
                throw new LocalizedException(
                    __('Not all of your products are available in the requested quantity.')
                );
            }
        }

        if (!empty($sourceItems)) {
            $this->sourceItemsSave->execute($sourceItems);
        }
    }
}