code: def normalize_value(self, value, transform=True):
          
          if transform:
              value = self.transform_value(value)
          return str(self.field.from_python(value))
docs: Prepare the given value to be stored in the index
  
          It first calls ``transform_value`` if ``transform`` is ``True``, then
          calls the ``from_python`` method of the field, then cast the result
          to a str.
  
          Parameters
          ----------
          value: any
              The value to normalize
          transform: bool
              If ``True`` (the default), the value will be passed to ``transform_value``.
              The returned value will then be used.
  
          Returns
          -------
          str
              The value, normalized