/**
 * Loads the set of event IDs the given customer has favorited, restricted
 * to the given candidate event IDs - a single batched query.
 *
 * @param int $customerId
 * @param int[] $eventIds
 * @return int[] Event IDs that are favorites
 */
public function getFavoriteEventIds(int $customerId, array $eventIds): array
{
    if ($eventIds === []) {
        return [];
    }

    $connection = $this->resourceConnection->getConnection();
    $select = $connection->select()
        ->from($this->resourceConnection->getTableName(self::TABLE_NAME), ['event_id'])
        ->where('customer_id = ?', $customerId)
        ->where('event_id IN (?)', $eventIds);

    return array_map('intval', $connection->fetchCol($select));
}

<?php

declare(strict_types=1);

namespace Mironsoft\Event\Model\Resolver\Batch;

use Magento\Authorization\Model\UserContextInterface;
use Magento\Framework\GraphQl\Query\Resolver\BatchRequestItemInterface;
use Magento\Framework\GraphQl\Query\Resolver\BatchResponse;
use Magento\Framework\GraphQl\Query\Resolver\BatchResolverInterface;
use Mironsoft\Event\Api\Data\EventInterface;
use Mironsoft\Event\Model\ResourceModel\EventFavorite;

/**
 * Batched resolver for the is_favorite field - collapses N per-event lookups
 * into a single query per GraphQL request.
 */
class IsFavorite implements BatchResolverInterface
{
    /**
     * @param EventFavorite $eventFavoriteResource Resource model for the favorites linkage table
     */
    public function __construct(
        private readonly EventFavorite $eventFavoriteResource,
    ) {
    }

    /**
     * Resolves is_favorite for every requested event in a single batch.
     *
     * @param BatchRequestItemInterface[] $requests One entry per Event in the current selection
     * @return BatchResponse
     */
    public function resolve(array $requests): BatchResponse
    {
        $response = new BatchResponse();

        $context = $requests !== [] ? $requests[0]->getContext() : null;
        $isCustomer = $context !== null
            && $context->getUserType() === UserContextInterface::USER_TYPE_CUSTOMER
            && $context->getExtensionAttributes()->getIsCustomer();

        if (!$isCustomer) {
            foreach ($requests as $request) {
                $response->addResponse($request, false);
            }

            return $response;
        }

        $eventIds = [];
        foreach ($requests as $request) {
            /** @var EventInterface|null $event */
            $event = ($request->getValue())['model'] ?? null;
            if ($event !== null && $event->getEventId() !== null) {
                $eventIds[] = $event->getEventId();
            }
        }

        $customerId = (int) $context->getUserId();
        $favoriteEventIds = $this->eventFavoriteResource->getFavoriteEventIds($customerId, $eventIds);

        foreach ($requests as $request) {
            /** @var EventInterface|null $event */
            $event = ($request->getValue())['model'] ?? null;
            $isFavorite = $event !== null && in_array($event->getEventId(), $favoriteEventIds, true);
            $response->addResponse($request, $isFavorite);
        }

        return $response;
    }
}