Prepare phase for session.

        :param session: sqlalchemy session

def prepare(self, session, event):
        """Prepare phase for session.

        :param session: sqlalchemy session
        """
        if not event:
            self.logger.warn("event empty!")
            return

        sp_key, sp_hkey = self._keygen(session)

        def _pk(obj):
            pk_values = tuple(getattr(obj, c.name)
                              for c in obj.__mapper__.primary_key)
            if len(pk_values) == 1:
                return pk_values[0]
            return pk_values

        def _get_dump_value(value):
            if hasattr(value, '__mapper__'):
                return _pk(value)
            return value
        pickled_event = {
            k: pickle.dumps({_get_dump_value(obj) for obj in objs})
            for k, objs in event.items()}
        with self.r.pipeline(transaction=False) as p:
            p.sadd(sp_key, session.meepo_unique_id)
            p.hmset(sp_hkey, pickled_event)
            p.execute()