code: def connect(self):
          
          if self.connected:
              return
          pk = self._pk
          if self.exists(pk=pk):
              self._connected = True
          else:
              self._pk = None
              self._connected = False
              raise DoesNotExist("No %s found with pk %s" % (self.__class__.__name__, pk))
docs: Connect the instance to redis by checking the existence of its primary
          key. Do nothing if already connected.