code: def clear(self, delete_thumbnails=False):
          
          prefix = settings.THUMBNAIL_KEY_PREFIX
          for key in self._find_keys_raw(prefix):
              self.cache.delete(key)
          KVStoreModel.objects.filter(key__startswith=prefix).delete()
          if delete_thumbnails:
              self.delete_all_thumbnail_files()
docs: We can clear the database more efficiently using the prefix here rather
          than calling :meth:`_delete_raw`.