Drop the given relation and cascade it appropriately to all
        dependent relations.

        :param _CachedRelation dropped: An existing _CachedRelation to drop.

def _drop_cascade_relation(self, dropped):
        """Drop the given relation and cascade it appropriately to all
        dependent relations.

        :param _CachedRelation dropped: An existing _CachedRelation to drop.
        """
        if dropped not in self.relations:
            logger.debug('dropped a nonexistent relationship: {!s}'
                         .format(dropped))
            return
        consequences = self.relations[dropped].collect_consequences()
        logger.debug(
            'drop {} is cascading to {}'.format(dropped, consequences)
        )
        self._remove_refs(consequences)