public async Task CanDecrementOrderItemQuantity()
{
Guid orderItemId = Guid.NewGuid();
int quantity = _fixture.Create <int>();
OrderItem orderItem = _fixture.Build <OrderItem>().With(a => a.Quantity, quantity).Create();
Mock <IShoppingCartApi> mockApi = new Mock <IShoppingCartApi>();
mockApi.Setup(a => a.GetOrderItemAsync(orderItemId, It.IsAny <CancellationToken>())).ReturnsAsync(orderItem);
mockApi.Setup(a => a.UpdateOrderItemAsync(orderItemId, quantity - 1, It.IsAny <CancellationToken>())).ReturnsAsync(new OrderItem(orderItem.Product, orderItem.Quantity - 1));
OrderItemManager sut = CreateSystemUnderTest(mockApi.Object);
OrderItem result = await sut.DecrementOrderItemQuantitytAsync(orderItemId);
Assert.Equal(orderItem.Quantity, result.Quantity);
}