The correct way to implement a transaction is to use the session object in each and every operation that we want to either commit or roll back at the end of it, as you can see from the following code:

   def tx_transfer_err(self, source_account, target_account, value):
       print(f'transferring {value} Hypnotons from {source_account} to {target_account}')
       with self.client.start_session() as ses:
           ses.start_transaction()
           res = self.accounts.update_one({'account_id': source_account}, {'$inc': {'balance': value*(-1)} }, session=ses)
           res2 = self.accounts.update_one({'account_id': target_account}, {'$inc': {'balance': value} }, session=ses)
           error_tx = self.__validate_transfer(source_account, target_account)

           if error_tx['status'] == True:
               print(f"cant transfer {value} Hypnotons from {source_account} ({error_tx['s_bal']}) to {target_account} ({error_tx['t_bal']})")
               ses.abort_transaction()
           else:
               ses.commit_transaction()

The only difference now is that we are passing session=ses in both of our update statements. In order to validate whether we have enough funds to actually make the transfer, we wrote a helper method, __validate_transfer, with its arguments being the source and target account IDs:

   def __validate_transfer(self, source_account, target_account):
       source_balance = self.accounts.find_one({'account_id': source_account})['balance']
       target_balance = self.accounts.find_one({'account_id': target_account})['balance']

       if source_balance < 0 or target_balance < 0:
          return {'status': True, 's_bal': source_balance, 't_bal': target_balance}
       else:
           return {'status': False}

Unfortunately, this attempt will also fail. The reason is the same as before. When we are inside a transaction, we make changes to the database that follow the ACID principles. Changes inside a transaction are not visible to any queries outside of it, until they are committed.