code: public function isPlaceholder($attributeName)
      {
        // Ref this model's attributes in a shorter variable.
        $def = $this->model_attributes[$attributeName];
  
        if (isset($def['models']) || (isset($def['model']) && isset($def['usesRefTable']))) {
          return true;
        }
        return false;
      }
docs: Sometimes a placeholder value of "1" will be loaded into models which indicate the data is available,
  but must be fetched if needed. This method indicates whether or not a value of 1 means it's a placeholder.
  
  @param $attributeName string An attribute name who's value in model_data is "1"
  @return bool