Cart page fatal error after creating a product collection plugin, Magento 2.3

Question

We have a need to sometimes fetch product prices from an API for products loaded on the frontend. We figured it would be easiest to create a plugin for the collection:

<?xml version="1.0"?>
<!-- .../Vendor/Module/etc/frontend/di.xml -->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    ...
    <type name="Magento\Catalog\Model\ResourceModel\Product\Collection">
        <plugin name="afterProductCollectionPlugin" type="Vendor\Module\Plugin\Catalog\Model\ResourceModel\Product\CollectionPlugin" />
    </type>
</config>

<?php
namespace Vendor\Module\Plugin\Catalog\Model\ResourceModel\Product;

use Magento\Catalog\Model\ResourceModel\Product\Collection;

class CollectionPlugin
{

    /**
     * @param Collection $subject
     * @param bool $printQuery
     * @param bool $logQuery
     * @return array
     */
    public function afterLoad(Collection $subject, $printQuery = false, $logQuery = false)
    {
        return [$printQuery, $logQuery];
    }
}

However after flushing the caches the /checkout/cart page throws a fatal error:

<b>Fatal error</b>:  Uncaught Error: Call to undefined method Magento\Catalog\Model\ResourceModel\Product\Link\Product\Collection\Interceptor::isComposite() in C:\xampp\htdocs\vendor\magento\module-catalog\view\frontend\templates\product\list\items.phtml:180
Stack trace:
#0 C:\xampp\htdocs\vendor\magento\framework\View\TemplateEngine\Php.php(59): include()
#1 C:\xampp\htdocs\vendor\magento\framework\View\Element\Template.php(271): Magento\Framework\View\TemplateEngine\Php-&gt;render(Object(Magento\Checkout\Block\Cart\Crosssell), 'C:/xampp/htdocs...', Array)
#2 C:\xampp\htdocs\vendor\magento\framework\View\Element\Template.php(301): Magento\Framework\View\Element\Template-&gt;fetchView('C:/xampp/htdocs...')
#3 C:\xampp\htdocs\vendor\magento\framework\View\Element\AbstractBlock.php(1097): Magento\Framework\View\Element\Template-&gt;_toHtml()
#4 C:\xampp\htdocs\vendor\magento\framework\View\Element\AbstractBlock.php(1101): Magento\Framework\View\Element\AbstractBlock-&gt;Magento\Framework\View\Element\{closure}()
#5 C:\xampp\htdocs\vendor\magen in <b>C:\xampp\htdocs\vendor\magento\module-catalog\view\frontend\templates\product\list\items.phtml</b> on line <b>180</b><br />

How to fix this?

Answer 1

In case you are using the after plugin, you must have a $result in the arguments first, and it must return the $result. See example:

<?php
namespace Vendor\Module\Plugin\Catalog\Model\ResourceModel\Product;

use Magento\Catalog\Model\ResourceModel\Product\Collection;

class CollectionPlugin
{

    /**
     * @param Collection $subject
     * @param bool $printQuery
     * @param bool $logQuery
     * @return array
     */
    public function afterLoad(Collection $subject, $result, $printQuery = false, $logQuery = false)
    {
        return $result;
    }
}

PS: more info in DevDocs