I am learning GraphQL server with PHP, MySQL and I have following problems with nested objects.

This is my ProductType.php:

<?php

namespace AppTypes;

use GraphQLTypeDefinitionType;
use GraphQLTypeDefinitionObjectType;
use AppServicesProductService;
use AppTypesCategoryType;
use AppConfigsDatabase;
use AppModelsCategory;
use RuntimeException;
// Es aris GraphQL type, amis gareshe fetch ar mushaobs
class ProductType extends ObjectType
{
    public function __construct()
    {
        // $categoryType = new CategoryType();
        $categoryType = new ObjectType([
            'name' => 'Category',
            'fields' => [
                'id' => Type::int(),
                'name' => Type::string(),
            ],
        ]);

        $config = [
            'name' => 'Product',
            'fields' => [
                'id' => Type::string(),
                'name' => Type::string(),
                'in_stock' => Type::boolean(),
                'description' => Type::string(),
                'brand' => Type::string(),
                'category_id' => Type::int(),
                'category' => [
                    'type' => $categoryType,
                    'resolve' => function ($product) {
                        // Log the category_id to ensure it is being passed correctly
                        error_log("Fetching category for product with category_id: " . $product['category_id']);

                        // Check if category_id is valid
                        if (!$product['category_id']) {
                            error_log("No category_id provided.");
                            return null;
                        }

                        $db = new Database();
                        $conn = $db->getConnection();

                        $stmt = $conn->prepare("SELECT id, name FROM categories WHERE id = ?");
                        if (!$stmt) {
                            error_log('Database prepare error: ' . $conn->error);
                            return null;
                        }

                        $stmt->bind_param('i', $product['category_id']);
                        $stmt->execute();

                        $result = $stmt->get_result();
                        if (!$result) {
                            error_log('Database query error: ' . $conn->error);
                            return null;
                        }

                        $category = $result->fetch_assoc();
                        error_log("Fetched category: " . json_encode($category));

                        return $category;
                    },
                ],
            ],
        ];

        parent::__construct($config);
    }
}

This is my Product.php:

<?php

namespace AppModels;

use AppConfigsDatabase;

class Product
{
    private $conn;

    public function __construct()
    {
        $database = new Database();
        $this->conn = $database->getConnection();
    }

    public function getProducts()
    {
        // Join products with categories to get category details
        $sql = "
            SELECT * from products
        ";

        // SELECT p.id, p.name, p.in_stock, p.description, p.brand,
        //            c.id AS category_id, c.name AS category_name
        //     FROM products p
        //     LEFT JOIN categories c ON p.category_id = c.id

        $result = $this->conn->query($sql);

        $products = [];
        while ($row = $result->fetch_assoc()) {
            $products[] = [
                'id' => $row['id'],
                'name' => $row['name'],
                'in_stock' => $row['in_stock'],
                'description' => $row['description'],
                // 'category' => [
                //     'id' => $row['category_id'],
                //     'name' => $row['category_name'],
                // ],
                'brand' => $row['brand'],
                'category_id' => $row['category_id'],
            ];
        }



        return $products;
    }
}

It doesn’t matter what I am trying to query or what code I write here, everytime I nest category on product, it returns a internal server error:

{
    "errors": [
        {
            "message": "Internal server error",
            "locations": [
                {
                    "line": 2,
                    "column": 5
                }
            ],
            "path": [
                "products",
                0
            ]
        }
    ],
    "data": {
        "products": [
            null
        ]
    }
}

I know this code looks messy and doesn’t seem too professional but maybe someone puts me on correct rails, how that works? What am I missing?