'The stream token for an event
Args:
event_id(str): The id of the event to look up a stream token for.
Raises:
StoreError if the event wasn\'t in the database.
Returns:
A deferred "t%d-%d" topological token.'

def get_topological_token_for_event(self, event_id):

    return self._simple_select_one(table='events', keyvalues={'event_id': event_id}, retcols=('stream_ordering', 'topological_ordering'), desc='get_topological_token_for_event').addCallback((lambda row: ('t%d-%d' % (row['topological_ordering'], row['stream_ordering']))))
