gistfile1.txt

$rels = $this->db->select('rel_parent')->where('rel_child', $userid)->where('rel_type', 'favourite')->from('content_relationships')->get();

$content_ids = '';
$i = 0;
foreach ($rels->result_array() as $row)
{
    if($rels->num_rows() === $i)
    {
        $content_ids .= $row['rel_parent'];
    }
    else
    {
        $content_ids .= $row['rel_parent'] . ',';
    }
    $i++;
}