我的数据库集合是这样的{"_id":"57f4c1323eb5c694041ghea3","example":[{"exId":"57f4c0d43eb5c694041fdebd","_id":ObjectId("57f4c1323eb5c694041fded2"),"projects":["57f4c1303eb5c694041fdec6"]},{"exId":"57f24ee56da92f9c19b0efd4","_id":ObjectId("57f4cfa93eb5c694041fdf4c"),"projects":["57f4cfa83eb5c694041fdf45"]},{"