如何在Laravel上创建多级类别并填充到select选项中

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateCategoriesTable extends Migration
{
    public function up()
    {
        Schema::create('categories', function (Blueprint $table) {
            $table->id();
            $table->string('name');
            $table->unsignedBigInteger('parent_id')->nullable();
            $table->timestamps();

            $table->foreign('parent_id')->references('id')->on('categories');
        });
    }

    public function down()
    {
        Schema::dropIfExists('categories');
    }
}

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    protected $fillable = ['name', 'parent_id'];

    public function children()
    {
        return $this->hasMany(Category::class, 'parent_id');
    }
}

// web.php
Route::get('/categories', 'CategoryController@index');

// CategoryController.php
namespace App\Http\Controllers;

use App\Models\Category;
use Illuminate\Http\Request;

class CategoryController extends Controller
{
    public function index()
    {
        $categories = Category::whereNull('parent_id')->with('children')->get();

        return view('categories.index', compact('categories'));
    }
}

<!-- categories/index.blade.php -->
<form>
    <div class="form-group">
        <label for="category">Category:</label>
        <select class="form-control" id="category" name="category">
            <option value="">Select Category</option>
            @foreach($categories as $category)
                <option value="{{ $category->id }}">{{ $category->name }}</option>
                @foreach($category->children as $child)
                    <option value="{{ $child->id }}">- {{ $child->name }}</option>
                @endforeach
            @endforeach
        </select>
    </div>
    <button type="submit" class="btn btn-primary">Submit</button>
</form>