code: def value_to_string(self, obj):
          
          value = self._get_val_from_obj(obj)
          return self.get_prep_value(value)
docs: Convert the field value from the provided model to a string.
  
          Used during model serialization.
  
          Args:
              obj: db.Model, model object
  
          Returns:
              string, the serialized field value