public void ChangeItemQuantity(ShoppingCart cart, ShoppingCartItem item, int newQuantity)
{
Require.NotNull(cart, "cart");
Require.NotNull(item, "item");
Require.That(newQuantity > 0, "newQuantity", "Quantity should be greater than zero.");
if (item.Quantity != newQuantity)
{
var oldQuantity = item.Quantity;
item.Quantity = newQuantity;
_repository.Database.SaveChanges();
Event.Raise(new CartItemQuantityChanged(cart, item, oldQuantity), _instance);
}
}