我一直在嘗試在MongoDB中使用MapReduce來做我認為簡單的過程。我不知道這是否是正確的方法，甚至不應該使用MapReduce。我用谷歌搜索了我想到的關鍵詞，并試圖在文檔中找到我認為最成功的文檔，但是沒有結果。也許我在想這個嗎？我有兩個收藏夾：details和gpasdetails由一堆文件（3+百萬個）組成。所述studentid元件可以被重復兩次，每個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。每個僅一個條目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，當我運行它時，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 } }我想念的類數組。另外，順便說一句，如何訪問生成的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？