mongo 多表级联

{"$lookup": { "from":"enroll", "localField":"enroll_id", "foreignField":"enroll_str_id", "as":"enroll_doc"} },

{"$unwind":"$enroll_doc"},

{"$lookup": { "from":"cefa_students", "localField":"enroll_doc.cefa_id", "foreignField":"cefa_id", "as":"cefa_stu_doc"} },

{"$unwind":"$cefa_stu_doc"},

{"$project":{"class":"$cefa_stu_doc.class", "grade":"$cefa_stu_doc.grade", "trade_id":"$trade_id", "enroll_name":"$enroll_name", "enroll_type":"$enroll_type","total_amount":"$total_amount", "class_name":"$class_name", "scene_name":"$scene_name", "order_dttm":"$order_dttm", "yearMonthDay": { $dateToString:{ format:"%Y-%m-%d", date:"$order_dttm"}}, "yearMonth": { $dateToString:{ format:"%Y-%m", date:"$order_dttm"}}}},