如果已经使用Laravel分配用户，如何限制同一用户在一个组中分配

php artisan make:middleware CheckUserGroupMiddleware

<?php

namespace App\Http\Middleware;

use Closure;
use Illuminate\Support\Facades\Auth;

class CheckUserGroupMiddleware
{
    public function handle($request, Closure $next)
    {
        $user = Auth::user(); // 获取当前认证用户

        // 检查用户是否已经在一个组中分配
        if ($user->group_id) {
            return response('You are already assigned to a group.', 403);
        }

        return $next($request);
    }
}

'checkUserGroup' => \App\Http\Middleware\CheckUserGroupMiddleware::class,

Route::post('/assign', 'AssignmentController@assign')
    ->middleware('checkUserGroup');