Nested triggers in SQL - asp.net

Question

I have the following tables: replies and messages. I would like to imitate Facebook's behavior: when a message is deleted, all related replies are deleted as well. My tables look like this:
REPLIES
messageId replyId
6b61d107-dff3-4374-a3a2-75ac7478a2f2 865c873d-0210-482a-b8bd-371c4f07f0cf
MESSAGES
id body
865c873d-0210-482a-b8bd-371c4f07f0cf this is the reply
6b61d107-dff3-4374-a3a2-75ac7478a2f2 this is the message
I have created one first trigger, which works, which deletes the related lines in Replies when a message is deleted. I now would like to create another trigger which would delete the related message every time a line in Replies is deleted. Right now the replies become messages of their own, which doesn't make sense. Here's the second trigger:
CREATE TRIGGER TRG_DEL_MESSAGES
ON Replies
FOR DELETE
AS
DELETE FROM Messages WHERE id = (SELECT replyId FROM DELETED)
Which brings the following error when I try to delete something:
Maximum stored procedure, function, trigger, or view nesting level exceeded (limit 32).
Apparently there's an infinite loop going and I think it is because the DELETED table is filled by data from the first trigger (which provoked the second). But really I'm not sure and I'd appreciate some help. Thanks!

Answer 1

Will something like this work for you? Remove the recursion by just using a single trigger on Messages, that calculates all of the related messages up front, so you should get at most one nested call? It deletes all the related messages from both messages and replies
CREATE TRIGGER TRG_DEL_REPLIES
ON [Messages]
FOR DELETE
AS
BEGIN
DECLARE #Related TABLE (MessageId uniqueidentifier)
--get all related messages so that we don't recurse
BEGIN
WITH AllReplies (MessageId)
AS
(
--Anchor
SELECT D.MessageId
FROM Deleted D
UNION ALL
--Recurse
SELECT R.ReplyId
FROM AllReplies AR
JOIN Replies R
ON AR.MessageId = R.MessageId
)
INSERT INTO #Related
SELECT *
FROM AllReplies
END
--delete the replies
DELETE R
FROM Replies R
JOIN #Related REL
ON R.MessageId = REL.MessageId
--delete the messages
DELETE M
FROM [Messages] M
JOIN #Related REL
ON REL.MessageId = M.MessageId
LEFT
JOIN DELETED D
ON REL.MessageId = D.MessageId
WHERE D.MessageId IS NULL
END
To translate this into a stored proc as you wanted, i would do this, rather than a loop that does multiple separate deletes:
CREATE PROCEDURE DeleteMessageWithReplies(#messageId uniqueidentifier)
AS
BEGIN
DECLARE #Related TABLE (MessageId uniqueidentifier)
--get all related messages
BEGIN
WITH AllReplies (MessageId)
AS
(
--Anchor
SELECT #messageId
UNION ALL
--Recurse
SELECT R.ReplyId
FROM AllReplies AR
JOIN Replies R
ON AR.MessageId = R.MessageId
)
INSERT INTO #Related
SELECT *
FROM AllReplies
END
--delete the replies
DELETE R
FROM Replies R
JOIN #Related REL
ON R.MessageId = REL.MessageId
--delete the messages that haven't already been deleted
DELETE M
FROM [Messages] M
JOIN #Related REL
ON REL.MessageId = M.MessageId
END

Answer 2

Thanks for your help Fergus, I appreciate it. However, as #Ben pointed, a stored procedure is easier and simpler to code. This is what I've just written, it could probably be improved but at least it works.
EXEC('CREATE PROCEDURE deleteMessageWithReplies(#messageId uniqueidentifier)
AS
BEGIN
DECLARE #repliesCount int
SELECT #repliesCount = (SELECT COUNT(*) FROM Replies WHERE messageId=#messageId)
DECLARE #cpt int
SET #cpt = 0
DELETE FROM Messages WHERE id = #messageId
WHILE #cpt < #repliesCount
BEGIN
DECLARE #replyId uniqueidentifier
SELECT #replyId = (SELECT TOP 1 replyId FROM Replies WHERE messageId=#messageId)
DELETE FROM Replies WHERE replyId = #replyId
DELETE FROM Messages WHERE id=#replyId
SET #cpt = #cpt+1
END
END')

Answer 3

I would recommend either generating all the deletes at once in a trigger, as Fergus Bown suggested, or moving your delete logic to a stored procedure call. In our application, we use the stored procedure approach for all CRUD operations (create, read, update, delete).
The downside is that a rookie support DBA might goof if they use SQL to delete a single Reply without deleting all the other Messages associated with it. But such a DBA ought to know to use the stored procedures instead (or get the SQL right in the first place).