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I am using Magento 2.3.0

I have some products in my website which allows custom printing and these products are assigned in category "Printing Products". Now I want to customize add to cart functionality for these custom printing products. I don't want to add any other category products with these custom printing products. For Ex., Let's say When there is custom printing product is already added to a product and if I try to add any products other than "Printing product" category then this product must not be added to cart and an error message should be displayed.

In short, Custom Printing category products are not allowed to shop with other products. So, How can I do this?

EDIT : When the cart is empty or cart is having other product from "Printing Products" category then I should be able to add printing product into the cart and in this situation, if I try to add products other than "Printing Products" category then the error message should be shown.

You can try the step below to restrict add to cart from diffrient categories.

Assume you are using a custom module name "Company_MyModule".

STEPP 1)

Create di.xml under YOUR-MAGENTO-ROOT/app/code/Company/MyModule/etc/

File: YOUR-MAGENTO-ROOT/app/code/Company/MyModule/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="MagentoCheckoutModelCart">
 <plugin name="restrictAddTocart"
 type="CompanyMyModuleModelPluginCart"/>
 </type>
</config>

Step 2)

Create Cart.php under YOUR-MAGENTO-ROOT/app/code/Company/MyModule/Model/Plugin

File: YOUR-MAGENTO-ROOT/app/code/Company/MyModule/Model/Plugin/Cart.php

<?php

namespace CompanyMyModuleModelPlugin;

<?php

namespace CompanyMyModuleModelPlugin;

class Cart

 protected $productRepository;
 protected $storeManager;
 protected $checkoutSession;
 protected $logger;

 const PRINT_CATEGORY_ID = 4; // set print category ID

 public function __construct( MagentoCatalogApiProductRepositoryInterface $productrepositoryInterface, MagentoStoreModelStoreManagerInterface $storeManager, MagentoFrameworkSessionSessionManagerInterface $checkoutSession, PsrLogLoggerInterface $loggerInterface )
 
 $this->productRepository = $productrepositoryInterface;
 $this->storeManager = $storeManager;
 $this->checkoutSession = $checkoutSession;
 $this->logger = $loggerInterface;
 
 public function beforeAddProduct( MagentoCheckoutModelCart $subject, $productInfo, $requestInfo = null )
 
 $productId = $productInfo->getData('entity_id');
 $type_id = $productInfo->getData('type_id');
 $product = $this->productRepository->getById($productId); 
 $categoryIds = $product->getCategoryIds();
 $this->logger->info("Current Product Category IDs",$categoryIds);

 if ( is_array($categoryIds) && !in_array(self::PRINT_CATEGORY_ID,$categoryIds) ) 
 throw new MagentoFrameworkExceptionLocalizedException(__('Since Print product exist in your cart, You only order from Print Category products'));
 return $this;
 

 return array(
 $productInfo,
 $requestInfo 
 );