protected function isRelation($field)
    {
        //get relation name from dot notation
        if (strpos($field, '.') !== false) {
            list($field, $columnName) = explode('.', $field);
        }
        $has_ones = singleton($this->objectClass)->has_one();
        //check if relation is present in has ones
        return isset($has_ones[$field]);
    }