how to store session cart value into database

here controller:

        public function getAddToCart(Request $request,$id)
        {
             $product = Product::find($id);
             $oldCart = Session::has('cart') ? Session::get('cart') : null;
             $cart = new Cart($oldCart);
             $cart->add($product, $product->id);

             $request->session()->put('cart',$cart);

              //dd($request->session()->get('cart'));   //check the data value flow
          return redirect()->back();
    }