'Returns mapping event_id -> state_group'

@cachedList(cached_method_name='_get_state_group_for_event', list_name='event_ids', num_args=1, inlineCallbacks=True)
def _get_state_group_for_events(self, event_ids):

    rows = (yield self._simple_select_many_batch(table='event_to_state_groups', column='event_id', iterable=event_ids, keyvalues={}, retcols=('event_id', 'state_group'), desc='_get_state_group_for_events'))
    defer.returnValue({row['event_id']: row['state_group'] for row in rows})
