[Catalyst] how to implement Transactions (over different db tables) in a Catalyst project

[kakimoto at tpg.com.au]

So, in the case of an exception, an automatic rollback will take place.
I have tested this in my application and it seems that way.

Can anyone confirm this? 
Thank you:)

> This is fairly easy to get wrong - txn_do forces you to have a block 
> 
> which comprises the transaction scope, or your code won't compile.
> 
> It will start a transaction at the start of the block, and commit it 
> 
> at the end if an exception hasn't been encountered during the  
> transaction
>