code: def prepare_value_for_storage(self, value, pk):
          
          value = super(TextRangeIndex, self).prepare_value_for_storage(value, pk)
          return self.separator.join([value, str(pk)])
docs: Prepare the value to be stored in the zset
  
          We'll store the value and pk concatenated.
  
          For the parameters, see BaseRangeIndex.prepare_value_for_storage