In some cases, it may be interesting to logically group transactions in a "master transaction".
Use case: A user purchases bundles of stuff with in-game coins.
- master transactions saves the bundle details, handle payment - sub-transactions saves individual items of the bundles
add this to Transaction.init()
self.subtransactions = 
iterate over the subtransactions engine in main commit, after to main transaction's transactors successfully ran.
save as useall