AFAIK, DM doesn't support nested transactions. Also AFAICT the only DB that
supports true nested transactions (natively via START TRANSACTION/BEGIN SQL
primitive) is SQL Server. MySQL supports them via different SQL primitives
called "savepoints". I don't think SQLite has them, but I'd bet that
Postgresql does, in some form or another.
I recently looked into why DM doesn't support them (while AR does), and I came
to two basic conclusions:
(1) Nested transaction support (on a per-adapter basis) is totally
possible by switching transaction SQL primitive based on the level of
transactional nesting level in effect for a given repository connection. For
example, in MySQL the first transaction would use a BEGIN (like it does now)
while subsequent (nested) transactions would instead use SAVEPOINT.
(2) The separation of DO (which currently manages transaction SQL
primitives) and DM (the layer that actually knows about query scope & nesting
level) makes this harder than it is in AR (which doesn't have this separation).
All that's necessary would be to track current transaction level on a given
repository connection, add a standard ID-tag for every transaction (for
savepoint references), and depending on nesting level switch between (in MySQL)
BEGIN vs. SAVEPOINT and their corollary COMMIT/ROLLBACK mechanisms.
I don't know if this is even on the current list of priorities, but it'd be
nice: I haven't found an easy way to use transactions in testing frameworks to
reset DB state quickly when doing functional testing of controller methods that
employ transactions themselves.
cheers,
--jordan
On Dec 28, 2009, at 8:24 AM, Kai Chen wrote:
> Hi all.
>
> I found could not use nested transaction any more in datamapper 0.10.x.
>
> And seems this commit force dm use regular transactions
> http://github.com/datamapper/dm-core/commit/1a0edcdccba4d9540b42bbd2cc5ae239fbe1d296
>
> So when you use start a transaction and modify data in other
> transaction, it will got this error `Lock wait timeout exceeded; try
> restarting transaction - (DataObjects::SQLError)`.
>
> --
> Regards,
> Kai Chen
>
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