use Illuminate\Support\Arr;
use Illuminate\Database\Eloquent\Relations\Relation;
use Illuminate\Support\Facades\Validator;

Validator::extend('poly_exists', function ($attribute, $value, $parameters, $validator) {
        // usage:
        // 'commentable_id' => 'required|poly_exists:commentable_type

        if (! $type = Arr:get($validator->getData(), $parameters[0], false)) {
            return false;
        }

        if (Relation::getMorphedModel($type)) {
            $type = Relation::getMorphedModel($type);
        }

        if (! class_exists($type)) {
            return false;
        }

        return ! empty(resolve($type)->find($value));
});