Prepares the bulk saver to load the trace graph info into the
        database.

def _prep_save(self):
        """ Prepares the bulk saver to load the trace graph info into the
        database.
        """
        log.info("Preparing bulk save.")
        self.graph.update_bulk_saver(self.bulk_saver)

        log.info(
            "Dropped %d unused preconditions, %d are missing",
            sum(len(v) for v in self.summary["precondition_entries"].values()),
            len(self.summary["missing_preconditions"]),
        )

        log.info(
            "Dropped %d unused postconditions, %d are missing",
            sum(len(v) for v in self.summary["postcondition_entries"].values()),
            len(self.summary["missing_postconditions"]),
        )
        del self.summary["postcondition_entries"]