Handling Constraint SqlException in Asp.net

Question

Hello,

Suppose I have a user table that creates strong relationships (Enforce Foreign Key Constraint) with many additional tables. Such orders table ..

If we try to delete a user with some orders then SqlException will arise.. How can I catch this exception and treat it properly?

Is this strategy at all?

1) first try the delete action if an exception Occur handel it?

2) Or maybe before the delete action using code adapted to ensure that offspring records throughout the database and alert according to .. This piece of work ...

So how to do it?

--Edit:

The goal is not to delete the records from the db! the goal is to inform the user that this record has referencing records. do i need to let sql to execute the delete command and try to catch SqlException? And if so, how to detect that is REFERENCE constraint SqlException?

Or - should I need to write some code that will detect if there are referencing records before the delete command. The last approach give me more but its a lot of pain to implement this kind of verification to each entity..

Thanks

Answer 1

Do you even really want to actually delete User records? Instead I'd suggest having a "deleted" flag in your database, so when you "delete" a user through the UI, all it does is update that record to set the flag to 1. After all, you wouldn't want to delete users that had orders etc.

Then, you just need to support this flag in the appropriate areas (i.e. don't show "deleted" users in the UI).

Edit:
"...but just for the concept, assume that i do want delete the user how do i do that?"
You'd need to delete the records from the other tables that reference that user first, before deleting the user record (i.e. delete the referencing records first then delete the referenced records). But to me that doesn't make sense as you would be deleting e.g. order data.

Edit 2:
"And if so, how to detect that is REFERENCE constraint SqlException?"
To detect this specific error, you can just check the SqlException.Number - I think for this error, you need to check for 547 (this is the error number on SQL 2005). Alternatively, if using SQL 2005 and above, you could handle this error entirely within SQL using the TRY...CATCH support:

BEGIN TRY
    DELETE FROM User WHERE UserId = @MyUserId
END TRY
BEGIN CATCH
   IF (ERROR_NUMBER() = 547)
        BEGIN
            -- Foreign key constraint violation. Handle as you wish
        END
END CATCH

However, I'd personally perform a pre-check like you suggested though, to save the exception. It's easily done using an EXISTS check like this:

IF NOT EXISTS(SELECT * FROM [Orders] WHERE UserId=@YourUserId)
    BEGIN
        -- User is not referenced
    END

If there are more tables that reference a User, then you'd need to also include those in the check.