ordering by a count on a column

I have three tables user, product and purchases. I am currently trying to create a user page but i am struggling to work out how i would give the user a list based on the category of the product they have purchased most.

USER: id, name Products: id, name, category Purchases: id, user_id, product_id

Here is what i've tried to bring back a counted list of the categories of products a user has purchased most:

 $scategory = Purchase::with('products')
            ->where('purchases.products_id', '=', 'products_id')
            ->Where('purchases.user_id', '=', Auth::user()->id)
            ->withCount('purchases.category')
                ->orderBy('purchases_category', 'DESC')
            ->get();

and i get this error

Call to undefined method Illuminate\Database\Query\Builder::products.category()

How do i get a list of the categories of products the user has purchased most?

via Chebli Mohamed