Why is DbContext.SaveChangesAsync throwing a "Subquery returned more than 1 value" error?

Question

The error in the title is occurring only in my QA environment right now, and not in my DEV environment. I've pointed my local machine towards the QA environment, ran a SQL trace, and can 1) reproduce the error and 2) see the actual SQL code being generated.

What I don't understand is how in the hell an error like this is even possible on an update? Here's my code:

            var investigation = await (
                        from  ti in DbContext.Investigation
                        where ti.Id == model.InvestigationId
                        select ti
                    ).SingleAsync();

            investigation.QueueId = model.QueueId;

            var result = await DbContext.SaveChangesAsync();

            return result;

I initially thought that first line (query) was returning more than one result, which would make sense. However, the error occurs on the var result = await DbContext.SaveChangesAsync(); line. Could someone please point me in the right direction to resolve this? My SQL query as picked-up by SQL Server Profiler:

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Investigation] SET [QueueId] = @p0
WHERE [Id] = @p1;
SELECT @@ROWCOUNT;

',N'@p1 bigint,@p0 smallint',@p1=7863,@p0=4

Answer 1

There is no subquery in this problem code as the error message suggests:

exec sp_executesql N'SET NOCOUNT ON;
UPDATE [Investigation] SET [QueueId] = @p0
WHERE [Id] = @p1;
SELECT @@ROWCOUNT;
',N'@p1 bigint,@p0 smallint',@p1=7863,@p0=4

Consequently, the error must be in trigger code fired by the UPDATE statement.

I see from your comment that extraneous INSERT statements were inadvertently included in the trigger code. A more common cause of these symptoms is that the trigger is improperly coded as to expect a single row to be affected. A trigger fires once per statement, not per row, so it is important that trigger code handle the possibility of multiple rows.