When not using entities as the api resources, in order to filter collections one or more custom filters can be applied to the classes annotated with the [#ApiResource] attribute.

Define a class as an ApiResource and annotate it with the desired filter(s), using the [#ApiFilter] attribute:

// src/ApiResource/ProductResource.php
<?php

declare(strict_types=1);

namespace App\ApiResource;

use App\Filter\ProductInStockFilter;
use ApiPlatform\Metadata\ApiFilter;
use ApiPlatform\Metadata\ApiResource;

[#ApiResource()]
[#ApiFilter(ProductInStockFilter::class)]
final class ProductResource

The custom filters should implement the \ApiPlatform\Doctrine\Orm\Filter\FilterInterface

<?php

declare(strict_types=1);

namespace App\Filter;

use Symfony\Component\HttpFoundation\Request;
use ApiPlatform\Doctrine\Orm\Filter\FilterInterface;

final class ProductInStockFilter implements FilterInterface
{
    public function apply(
        QueryBuilder $queryBuilder,
        QueryNameGeneratorInterface $queryNameGenerator,
        string $resourceClass,
        Operation $operation = null,
        array $context = [],
    ): void {
        if (!isset($context['filters']['product_in_stock'])) {
            return;
        }

        if ($context['filters']['product_in_stock'] === 'true') {
            $queryBuilder->andWhere('o.inStock > 1');
        }
    }

    // This function is only used to hook in documentation generators (supported by Swagger and Hydra)
    public function getDescription(string $resourceClass): array
    {
        return [
            'product_in_stock' => [
                'property' => 'inStock',
                'type' => 'bool',
                'required' => false,
            ],
        ];
    }
}

If utilising the default collection provider in your custom provider class then the query will be automatically altered to include the additional andWhere clause. Alternatively you can conditionally add the logic to your database lookup (repository method) to apply in your provider.