private function _getJunctionTableColumns($relationName, $model)
    {
        $junctionTableColumns = [];
        if (array_key_exists($relationName, $this->_relationsExtraColumns)) {
            if (is_callable($this->_relationsExtraColumns[$relationName])) {
                $junctionTableColumns = $this->_relationsExtraColumns[$relationName]($model);
            } elseif (is_array($this->_relationsExtraColumns[$relationName])) {
                $junctionTableColumns = $this->_relationsExtraColumns[$relationName];
            }
            if (!is_array($junctionTableColumns)) {
                throw new RuntimeException(
                    'Junction table columns definition must return an array, got ' . gettype($junctionTableColumns)
                );
            }
        }
        return $junctionTableColumns;
    }