how to calculate sum in laravel query code example

Example 1: laravel eloquent sum column

Product::where('id',$id)->sum('quantity');

Example 2: laravel sum column values

Auth::user()->products->sum('price');
// And on your App/User.php you need someting like this:
public function products(){
  return Products::where('id_buyer', Auth::user()->id)->get();
}