Php 停止转义列名codeigniter活动记录

我有以下activerecord代码来获取发票及其付款总额

$this->db->select('invoice.*,customer.custid,customer.cust_name,customer.cperson,customer.cust_email,customer.cust_mobile,customer.cust_address,sum(payment_received.amount) as paid_amount');
            $this->db->join('customer','customer.custid=invoice.custid');
            $this->db->join('payment_received','invoice.inid=payment_received.inid','left');
    $this->db->where('@paid_amount IS NULL');

由于codeigniter转义列名带有“`”this char，因此在“where子句”中显示错误“未知列”@paid_amount”

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如何停止转义列名？？？

您看过用户指南了吗？get和where方法都接受一个参数，以防止CI转义数据