如何在Laravel中在一个视图中创建一条记录和他的多个关系？

namespace App;

use Illuminate\Database\Eloquent\Model;

class User extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class);
    }
}

namespace App;

use Illuminate\Database\Eloquent\Model;

class Role extends Model
{
    public function users()
    {
        return $this->belongsToMany(User::class);
    }
}

<form action="{{ route('users.store') }}" method="POST">
    @csrf

    <label for="name">用户名:</label>
    <input type="text" name="name" id="name">

    <label for="roles">角色:</label>
    <select name="roles[]" id="roles" multiple>
        @foreach ($roles as $role)
            <option value="{{ $role->id }}">{{ $role->name }}</option>
        @endforeach
    </select>

    <button type="submit">创建</button>
</form>

namespace App\Http\Controllers;

use App\User;
use Illuminate\Http\Request;

class UserController extends Controller
{
    public function store(Request $request)
    {
        $user = User::create([
            'name' => $request->input('name'),
        ]);

        $user->roles()->attach($request->input('roles'));

        return redirect()->route('users.index');
    }
}