在laravel中加载双向递归关系

// Category.php
class Category extends Model
{
    public function parent()
    {
        return $this->belongsTo(Category::class, 'parent_id');
    }

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

// Subcategory.php
class Subcategory extends Model
{
    public function parent()
    {
        return $this->belongsTo(Subcategory::class, 'parent_id');
    }

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

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')->onDelete('cascade');
});

$category = Category::with('children')->find($categoryId);

$subcategory = Subcategory::with('parent')->find($subcategoryId);

$category = Category::with(['children' => function ($query) {
    $query->whereNull('parent_id');
}])->find($categoryId);

$category = Category::with(['children' => function ($query) {
    $query->limit(10); // 限制最多加载10层子分类
}])->find($categoryId);

$category = Category::with(['children' => function ($query) {
    $query->select('id', 'name', 'parent_id as parent_id_alias');
}])->find($categoryId);