Rails - Nov 2006 - understanding rails transactions support.

Hi,

I''m a bit confused about Rails transactions.  Are these Rails
transactions:

Model.transaction do
...
end

or does the above code actually start a database transaction?  If this
starts a db transaction, what sort of db transaction does this start?  I
would not want serializable transactions as this would be expensive.  If
this starts a Rails application level txn, is there a way to start a db
level txn and why would anybody use an application level txn if something
like this exists in Rails.

Thanks,

Sam
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