def test_change_quantity_for_product
    @cart = carts(:first)
    @item = @cart.line_items[0]
    assert_no_difference @cart.line_items, :count do
      assert_difference @item, :quantity, 1, true do
        assert_difference @cart, :total_in_cents, @item.price_in_cents do
          @cart.update_products(@item.product.id => 1)
        end
      end
    end
    assert_total
  end