Question

I have a pending order table with a check constraint to prevent people from ordering an item we don’t have in stock. This required me to create a counter function to decide if an insert can happen or not. It works until there is 1 item left in inventory then I get a message that we are out of stock of the item. I thought it was a dirty read issue but even after interducing a ReadPast hint I still see this behavior. Is there some other factor causing this problem? Or do I need to setup the isolation level differently?

I have tried calling this function with the sprokID and it returns true which is why I am thinking during insert there is a dirty read taking place.

ALTER TABLE [dbo].[PendingSprokOrders]  WITH CHECK ADD  CONSTRAINT [CK_SprokInStock] CHECK  (([dbo].[SprokInStockCount]([SprokID])=(1)))

FUNCTION [dbo].[SprokInStockCount] ( @SprokId INT )
RETURNS INT
AS 
    BEGIN 

        DECLARE @Active INT
        SET @Active = ( SELECT  COUNT(*)
                        FROM    [PendingSprokOrders] AS uac   WITH(READPAST)
                        WHERE   uac.SprokID = @SprokId
                      ) 
        DECLARE @Total INT
        SET @Total = ( SELECT
                                ISNULL(InStock, 0)
                       FROM     SprokInvetory
                       WHERE    id = @SprokId
                     )


        DECLARE @Result INT

        IF @Total - @Active > 0 
            SET @Result = 1
        ELSE 
            SET @Result = 0  
        RETURN @Result;
    END;

Answer 1

The math is off. Instead of:

    IF @Total - @Active > 0 
        SET @Result = 1
    ELSE 
        SET @Result = 0  

it should be:

    IF @Total - @Active > -1 
        SET @Result = 1
    ELSE 
        SET @Result = 0  

That’s because your constraint function can see the row that you are attempting to add and is counting it.