Update : sorry for the confusion, the second linq statement for emWeight is redundant.

Hi Cetan, Naomi. Many thanks for your help.

I had this originally, but it breaks at runtime when there are not EMTransactions (which can happen).

        public static decimal ComputeWeightDifference(int transactionHeaders_Id,PICSEntities context)
        {
            decimal? emWeight = (decimal)context.EMTransactions.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight);
            decimal? detailsWeight = 0;
            // emWeight = (decimal?) context.EMTransactions.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight) ?? 0;
            detailsWeight = context.TransactionDetails.Where(r => (r.TransactionHeaders_Id == transactionHeaders_Id)).Sum(r => r.Weight);
            return emWeight ?? 0  - detailsWeight ?? 0;
        }

I guess in a "if-you-can't-beat'm-join'm" frame of mind I will split it into the query and the summing. So much for the functional programming fans. :)