我有两个mongo集合，它们是关系：
contacts_collecion ：
{ "contact_id" : "1", "e" : "a1@a.aaa" }
{ "contact_id" : "2", "e" : "a2@a.aaa" }
{ "contact_id" : "3", "e" : "a3@a.aaa" }
...
...
contacts_groups_collection ：
{ "contact_id" : "2", "group_id" : "1" }
{ "contact_id" : "3", "group_id" : "1" }
{ "contact_id" : "3", "group_id" : "2" }
...
...
如此典型的多对多关系 - 我还有一个groups_collection。每个联系人都可以分为多个小组。
当我查询时：
db.contacts.aggregate([{ "$match" : { "a" : false, "e" : /ab/ } }, { "$lookup" : { "from" : "contacts_groups", "localField" : "contact_id", "foreignField" : "contact_id", "as" : "details" } }])
我明白了：
1) { "contact_id" : "1", "e" : "abcd@gmail.com", "details" : [{ "contact_id" : "1", "group_id" : "1" }, { "contact_id" : "1", "group_id" : "2" }] }
2) { "contact_id" : "2", "e" : "abcdef@gmail.com", "details" : [{ "contact_id" : "2", "group_id" : "2" }, { "contact_id" : "2", "group_id" : "3" }] }
3) { "contact_id" : "3", "e" : "abcdefgh@gmail.com", "details" : [] }
但我想只获得第二行（2），所以只有那些“group_id”：“3”。
我怎样才能做到这一点？
我这样做的方式是“加入和最有效”吗？