Sorting product Sort by category | WinterCMS/ OctoberCMS Plugin Builder

Another question from a noob.
On the page, I need to get a list of Categories with Subcategories something like:

• Category 1
  • Subcategory1_Category1
  • Subcategory2_Category1
• Category 2
  • Subcategory1_Category2
  • Subcategory2_Category 2

I have two models - Category and Subcategory.

I'm added relationship Subcategory Model:

use \Winter\Storm\Database\Traits\SimpleTree;
public $belongsTo = [
        'category' => [
            'AName\PName\Models\Category',
            'key' => 'category_id']
    ];

Catalog Components:

$this->page['categories'] = Category::get();
$subcategories = $this->page['subcategories'] = Subcategory::where('category_id', $category->$id)->get();

I receive 'Undefined variable: category'
Where is the mistake?
Thanks in advance.

You can use this approach.

You need to define sub_categories relation in to your parent model Category

// Category Model
public $hasMany = [
    'sub_categories' => AName\PName\Models\Subcategory
]

Now once you get the main categories you can retrieve its sub-category by just calling sub_categories relation

In code section

$this->page['categories'] = Category::get();

In markup section

{% for category in categories %}
    - {{ category.title }}
    {% for subCategory in category.sub_categories %}
        -- {{ subCategory.title }}        
    {% endfor %}
{% endfor %}

If any doubt please comment.