method not exist:think\db\Query->orderRaw

            $field         = Loader::parseName(substr($method, 5));
            $where[$field] = $args[0];
            return $this->where($where)->find();
        } elseif (strtolower(substr($method, 0, 10)) == 'getfieldby') {
            // 根据某个字段获取记录的某个值
            $name         = Loader::parseName(substr($method, 10));
            $where[$name] = $args[0];
            return $this->where($where)->value($args[1]);
        } else {
            throw new Exception('method not exist:' . __CLASS__ . '->' . $method);
        }
    }

    /**
     * 获取当前的数据库Connection对象
     * @access public
     * @return Connection
     */
    public function getConnection()