如何在CodeIgniter中从具有相同列名的表中输出数据?

这是我的疑问:

$query = $this->db->query('
    SELECT archives.id, archives.signature, type_of_source.description, media_type.description, origin.description 
    FROM archives, type_of_source, media_type, origin                                                             
    WHERE archives.type_of_source_id = type_of_source.id                                                          
    AND type_of_source.media_type_id = media_type.id                                                              
    AND archives.origin_id = origin.id                                                                            
    ORDER BY archives.id ASC
');

但是如何输出结果呢?这有效,但只得到最后一个描述(origin.description):

foreach ($query->result_array() as $row)
{
    echo $row['description'];
}

这行不通:

foreach ($query->result_array() as $row)
{
    echo $row['type_of_source.description'];
}

或者我应该重命名列(例如type_of_source_description)吗?