mongodb多密钥聚合

我有sql背景，了解mongodb中聚合数据的语法，但在mongodb中“展平”多键聚合的输出时遇到了麻烦。例如，标准语法如下所示：

db.transactions.aggregate(
    [
     { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } }
    ]
);

但这会以以下形式返回数据：

[{"_id":{"category":"Fees","postdate":"2013-01-04T05:00:00.000Z"},"total":24},
{"_id":{"category":"Fees","postdate":"2012-12-20T05:00:00.000Z"},"total":-0.02}]

我想要的是以下格式的数据，其中我仍然在两列或更多列上分组：

备选案文1：

[{"_id":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"_id":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "_id":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

备选案文2：

["category":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"category":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "category":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

---

如何在mongodb中实现这一点？

使用

备选案文1：

[{"_id":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"_id":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "_id":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

备选案文2：

["category":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"category":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "category":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

---

使用

备选案文1：

[{"_id":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"_id":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "_id":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

备选案文2：

["category":"Auto","postdate":"2013-01-04T05:00:00.000Z","total":24},
{"category":"Fees","postdate":"2012-12-20T05:00:00.000Z","total":-0.02}]

db.transactions.aggregate(
        [
         { $group: { _id: {"category":"$category","postdate":"$postdate"} , "total": { $sum: "$total" } } },
         { $project: { "_id":0, "category":"$_id.category", "postdate":"$_id.postdate", "total":1 } }
        ]
);

---

选项1第一行应为“费用”。那是打字错误吗？不，第二个应该是“自动”。换了。谢谢，我不明白。您是否只希望类别发生变化？你能提供样品文件吗？您在聚合中按类别分组，后跟发布日期。你还需要一个小组阶段来按类别对他们进行分组。你下面的回答回答了这个问题。那只是个打字错误。我对类别有不同的值。选项1第一行应该是“费用”。那是打字错误吗？不，第二个应该是“自动”。换了。谢谢，我不明白。您是否只希望类别发生变化？你能提供样品文件吗？您在聚合中按类别分组，后跟发布日期。你还需要一个小组阶段来按类别对他们进行分组。你下面的回答回答了这个问题。那只是个打字错误。我对类别有不同的值。