A simple Silverstripe 4 GraqhQL QueryCreator for returning a single DataObject by some unique field.

ByUrlSegmentQueryCreator.php

<?php
namespace SmartCookie\GraphQL;

use GraphQL\Type\Definition\ResolveInfo;
use GraphQL\Type\Definition\Type;
use SilverStripe\GraphQL\Manager;
use SilverStripe\GraphQL\OperationResolver;
use SilverStripe\GraphQL\QueryCreator;
use SilverStripe\ORM\DataObject;
use SilverStripe\GraphQL\Scaffolding\Util\ScaffoldingUtil;
use ReflectionClass;

/**
 * Creates a GraphQL Query that returns a DataObject by its URL Segment.
 */
class ByUrlSegmentQueryCreator extends QueryCreator implements OperationResolver
{
    /**
     * A Singleton of the object we want to serve.
     * @var SilverStripe\ORM\DataObject
     */
    private $do;

    /**
     * The Short name to use for naming our methods.
     * @var string
     */
    private $shortName;

    /**
     * @param SilverStripe\GraphQL\Manager $manager
     * @param string $doClass   DataObject class name that this query will fetch.
     * @param string $shortName Optional suffix to give to the query name. Defaults to the DataOobject classname without
     *                          the namespace.
     */
    public function __construct(Manager $manager = null, $doClass, $shortName = false)
    {
        // Create a singleton instance of our targeted DataObject class.
        $this->do = DataObject::singleton($doClass);

        // If no short name has been provided, let's infer one from our DO class.
        if ($shortName) {
            $this->shortName = $shortName;
        } else {
            $reflect = new ReflectionClass($this->do);
            $this->shortName = $reflect->getShortName();
        }

        parent::__construct($manager);
    }

    public function attributes()
    {
        return [
            'name' => 'byUrl' . $this->shortName,
            'description' => sprintf('Retrieve a single %s by its URL segment.', $this->do->singular_name())
        ];
    }

    public function args()
    {
        return [
            'UrlSegment' => ['type' => Type::nonNull(Type::string())]
        ];
    }

    public function type()
    {
        $typeName = ScaffoldingUtil::typeNameForDataObject(get_class($this->do));
        return $this->manager->getType($typeName);
    }

    public function resolve($object, array $args, $context, ResolveInfo $info)
    {
        $do = $this->do->get()->filter($args)->First();

        // Make sure the current user is allowed to view this specific DO
        if ($do && !$do->canView($context['currentUser'])) {
            throw new \InvalidArgumentException(sprintf(
                '%s view access not permitted',
                $this->do->getClassName()
            ));
        }

        return $do;
    }
}

graphql.yml

SilverStripe\GraphQL\Controller:
  schema:
    scaffolding_providers:
      - SmartCookie\Models\CarModel
      - SmartCookie\Models\Brand
    queries:
      byUrlCarModel: 'SmartCookie\GraphQL\ByUrlSegmentQueryCreator("SmartCookie\\Models\\CarModel")'
      byUrlBrand: 'SmartCookie\GraphQL\ByUrlSegmentQueryCreator("SmartCookie\\Models\\Brand")'