Connect the instance to redis by checking the existence of its primary
        key. Do nothing if already connected.

def connect(self):
        """
        Connect the instance to redis by checking the existence of its primary
        key. Do nothing if already connected.
        """
        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))