code: def get_version(self, extra=None):
          
  
          if extra:
              key = self._get_extra_key(extra)
          else:
              key = self.key
  
          v = self._get_cache(key).get(key)
          if v == None:
              v = self._increment_version(extra=extra)
  
          return "%s.%s" % (key, v)
docs: This will return a string that can be used as a prefix
          for django's cache key. Something like key.1 or key.1.2
  
          If a version was not found '1' will be stored and returned as
          the number for that key.
  
          If extra is given a version will be returned for that value.
          Otherwise the major version will be returned.
  
          :param extra: the minor version to get. Defaults to None.