Get Sub Categories with Parent category in the same request

Question

In my migration, I made column called parent_id and it takes 0 if this category is a parent category else it takes the id of its parent.

what I want to do is in one request I want to fetch all parent categories and inside each parent array of its sub-category

public function index()
{
    return Category::where('parent_id', '0')->each(function ($parent) {
        Category::where('parent_id', $parent->id)->get();
    })->get();
}