Last Insert id codeigniter

Model : 

$sql=$this->db->query("INSERT INTO `enquiry` (`code`,`name`,`email`,`phone`,`country`,`description`,`created_dt`)
VALUES (
'".$this->db->escape_str($this->input->post('code'))."',
'".$this->db->escape_str($this->input->post('name'))."',
'".$this->db->escape_str($this->input->post('email'))."',
'".$this->db->escape_str($this->input->post('phone'))."',
'".$this->db->escape_str($this->input->post('country'))."',
'".$this->db->escape_str($this->input->post('description'))."',
            Now()
)");

        $insert_id = $this->db->insert_id();
        return  $insert_id;