当视图模型关联规则：public $viewFields不为空时，$this->add()后的SQL语句为：INSERT INTO cooperation_v2_task cooperation_v2_task LEFT JOIN patient patient ON cooperation_v2_task.pat_id=patient.user_id LEFT JOIN doctors doctors ON cooperation_v2_task.doc_id=doctors.user_id (`create_time`) VALUES (1438749638)

当视图模型关联规则：public $viewFields为空时，$this->add()后的SQL语句为：INSERT INTO (`create_time`) VALUES (1438750038)

如何才能在视图模型中插入单张表的数据？