codeigniter加入表两次不起作用

Question

我不确定我在这里缺少什么，但这里是代码

    public function searchFlightsList($filters, $rpp, $pn, $sort_by, $sort) {

    $arr = array();

    $this->db->select('flight.id,' . 
        'flight_no, airline.name as airline_name, des.name as destination_name,' .
        'ori.name as origin_name,  time', false);
    $this->db->from($this->tbl_flight);
    $this->db->join($this->tbl_airline , 'airline.id = flight.airline_id', 'left');
    $this->db->join($this->tbl_location . ' as ori', 'ori.id = flight.origin_id', 'left');
    $this->db->join($this->tbl_location . ' as des', 'des.id = flight.destination_id', 'left');

    $this->db->order_by($sort_by, $sort);

    $query = $this->db->get();
    $arr = $query->result_array();
    return $arr;
}

上面的功能正常但是这个：

    public function searchPolicyList($filters, $rpp, $pn, $sort_by, $sort, $policy_no) {

    $arr = array();

    $this->db->select('policy.id,' .
        'CONCAT_WS(" ", is_main_id.first_name, is_main_id.middle_name, is_main_id.last_name) as main_full_name,' . 
        'policy_no', false);
    $this->db->from($this->tbl_policy);
    $this->db->join($this->tbl_user. ' as is_other_id','is_other_id.id = policy.associated_id','left');
    $this->db->join($this->tbl_user. ' as is_main_id','is_main_id.id = policy.main_id','left');             
    $this->db->order_by($sort_by, $sort);

    $query = $this->db->get();
    $arr = $query->result_array();
    return $arr;
}

产生致命错误：在非对象上调用成员函数result_array（）...

我检查了数据库表并且字段很好，但我只能使用associated_id和main_id字段加入用户

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