how to use order by codeigniter

am trying to use order by but i don't get all the results in table.
i have payment table 

Code:

payments={
          paymentid|academicID| studentID| invoiceID| amount
             1     |     1    |     1    |     1    |   2000  
             1     |     1    |     1    |     2    |    300  
             1     |     1    |     1    |     1    |     5  
             1     |     1    |     1    |     3    |    600
             1     |     1    |     1    |     3    |     10 

}

 and i have these two scripts
MY_model.php

PHP Code:

function get_order_by($array=NULL) {
        if($array != NULL) {
            $this->db->select()->from($this->_table_name)->order_by($this->_order_by)->where($array);
            $query = $this->db->get();
            return $query->result();
        } else {
            $this->db->select()->from($this->_table_name)->order_by($this->_order_by);
            $query = $this->db->get();
            return $query->result();
        }
    } 


action_helper.php

PHP Code:

function joide($array, $value, $key=NULL) {
    $returnArray = array();
    if(count($array)) {
        foreach ($array as $item) {
            if($key != NULL) {
                $returnArray[$item->$key] = strtolower($value) == 'obj' ? $item : $item->$value;
            } else {
                $returnArray[] = $item->$value;
            }
        }
    }
    return $returnArray;
} 


amount_m.php

PHP Code:

function get_order_by_amount($array=NULL) {
        $query = parent::get_order_by($array);
        return $query;
    } 


controller.php

PHP Code:

$payments = joide($this->amount_m->get_order_by_amount(['academicID' => $academicID]), 'obj', 'invoiceID');

$this->data['amount'] = $payments; 


so when i run  <?php dd($mummy) ?> i don't get all the results,  i mean it won't get the amount = 3 and 10 where invoiceid =1 and 3 respectively but it returns  the 2000, 3000 and 600, it seems it doesn't return the amount where invoiceID is the same like for 1 and 3