Laravel | Fetch Count of Items in Categories

I have following 2 Tables:

Ad: {id, title, category_id} Category: {id, name, parent_id}

Table 1: Ad

---|----------------------------|------------
id | title                      | category_id
---|----------------------------|------------
1  | Ad in Cars category        | 2
2  | Another Ad in Cars category| 2
3  | Ad in Vehicles category    | 1 

Table 2: Category

---|-----------|-----------
id | name      | parent_id
---|-----------|-----------
1  | Vehicles  | NULL
2  | Cars      | 1 

Now I need to fetch Categories and count of ads in each category. Also, if a category has ads in child categories, then the count of ads for a parent category should also include ads belonging to child category.

So if I fetch the count of ads in Vehicle category, I should get:

Category | Count
---------|-----------
Vehicles | 3

And if I fetch count of ads in Cars Category, I should get:

Category | Count
---------|-----------
Cars     | 2

How do I achieve this in Laravel? I'm using Laravel 5.4.

Thanks!

via Rishab