public void ShouldChangeQuantityOfItemInBasketGivenExistingItemDetail()
{
//arrange
var customerId = 123456789;
var itemDetail = new
{
ProductId = 1234,
Quantity = 1
};
var newQuantity = 25;
var sut = new CustomerBasket(customerId);
sut.AddItem(itemDetail.ProductId, itemDetail.Quantity);
//act
sut.ChangeQuantity(itemDetail.ProductId, newQuantity);
//assert
var result = sut.Items.Single();
Assert.Equal(itemDetail.ProductId, result.ProductId);
Assert.Equal(newQuantity, result.Quantity.Amount);
}