custom validator in form oject

WidgetCreateRequest.php

<?php

namespace App\Http\Requests;

use Illuminate\Foundation\Http\FormRequest;
use Illuminate\Http\Request;
use App\Category;
use App\Subcategory;
use Illuminate\Validation\Factory as ValidationFactory;

class WidgetCreateRequest extends FormRequest
{

    public function __construct(ValidationFactory $validationFactory)
    {

        $validationFactory->extend(
            'belongsToCategory',
            function ($attribute, $value, $parameters) {

                $category_id = request()->input('category_id');

                $verified = Subcategory::where('id', $value)
                        ->where('category_id', $category_id)
                        ->exists();


                return 'belongsToCategory' == $verified;
            },
            'Sorry, subcategory does not belong to category!'
        );

    }
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {

        $count = Category::count();


        return [
            'name' => 'required|unique:widgets|string|max:30',
            'category_id' => "required|numeric|digits_between:1,$count",
            'subcategory_id' => 'required|belongsToCategory'
        ];
    }
}