我一直在嘗試在MongoDB中使用MapReduce來(lái)做我認(rèn)為簡(jiǎn)單的過(guò)程。我不知道這是否是正確的方法，甚至不應(yīng)該使用MapReduce。我用谷歌搜索了我想到的關(guān)鍵詞，并試圖在文檔中找到我認(rèn)為最成功的文檔，但是沒(méi)有結(jié)果。也許我在想這個(gè)嗎？我有兩個(gè)收藏夾：details和gpasdetails由一堆文件（3+百萬(wàn)個(gè)）組成。所述studentid元件可以被重復(fù)兩次，每個(gè)year，如下所示：{ "_id" : ObjectId("4d49b7yah5b6d8372v640100"), "classes" : [1,17,19,21], "studentid" : "12345a", "year" : 1}{ "_id" : ObjectId("4d76b7oij7s2d8372v640100"), "classes" : [2,12,19,22], "studentid" : "98765a", "year" : 1}{ "_id" : ObjectId("4d49b7oij7s2d8372v640100"), "classes" : [32,91,101,217], "studentid" : "12345a", "year" : 2}{ "_id" : ObjectId("4d76b7rty7s2d8372v640100"), "classes" : [1,11,18,22], "studentid" : "24680a", "year" : 1}{ "_id" : ObjectId("4d49b7oij7s2d8856v640100"), "classes" : [32,99,110,215], "studentid" : "98765a", "year" : 2}...gpas具有與相同studentid的元素details。每個(gè)僅一個(gè)條目studentid，如下所示：{ "_id" : ObjectId("4d49b7yah5b6d8372v640111"), "studentid" : "12345a", "overall" : 97, "subscore": 1}{ "_id" : ObjectId("4f76b7oij7s2d8372v640213"), "studentid" : "98765a", "overall" : 85, "subscore": 5}{ "_id" : ObjectId("4j49b7oij7s2d8372v640871"), "studentid" : "24680a", "overall" : 76, "subscore": 2}...但是，當(dāng)我運(yùn)行它時(shí)，這是我得到的集合：{ "_id" : "12345a", "value" : { "studentid" : "12345a", "classes_1" : [ ], "classes_2" : [ ], "overall" : 97, "subscore" : 1 } }{ "_id" : "98765a", "value" : { "studentid" : "98765a", "classes_1" : [ ], "classes_2" : [ ], "overall" : 85, "subscore" : 5 } }{ "_id" : "24680a", "value" : { "studentid" : "24680a", "classes_1" : [ ], "classes_2" : [ ], "overall" : 76, "subscore" : 2 } }我想念的類(lèi)數(shù)組。另外，順便說(shuō)一句，如何訪(fǎng)問(wèn)生成的MapReduce value元素中的元素？MapReduce總是輸出到value還是您將其命名為其他名稱(chēng)？