<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def __prepare_bloom(self):
 """Prepare bloom for existing checks """

        self.__bloom = pybloom_live.ScalableBloomFilter()
        columns = [getattr(self.__table.c, key) for key in self.__update_keys]
        keys = select(columns).execution_options(stream_results=True).execute()
        for key in keys:
            self.__bloom.add(tuple(key))