/// <summary>
/// adds an item to a cart
/// </summary>
/// <param name="id"></param>
/// <param name="item"></param>
/// <exception cref="InvalidOperationException">Thrown if cart is already closed</exception>
/// <returns></returns>
public async Task AddToCart(string id, CartItem item)
{
var db = _redisProvider.Database;
var closed = bool.Parse(await db.HashGetAsync(CartKey(id), "Closed"));
if (closed)
{
throw new InvalidOperationException("Cart has already been closed out");
}
var key = CartKey(id);
await db.HashSetAsync(key, item.AsHashEntries(CartItemKey(id, item.Isbn)).ToArray());
}