Python Pymongo和聚合框架查询

我试着用pymongo编写聚合查询，但我自己做不到，我试过文档，试过谷歌，试过堆栈溢出，我就是找不到答案。我的数据样本：

{"_id": id, "site": "site A", "weekday": 1, "value": 1}
{"_id": id, "site": "site B", "weekday": 2, "value": 0}
{"_id": id, "site": "site C", "weekday": 3, "value": 1}
{"_id": id, "site": "site A", "weekday": 2, "value": 0}
{"_id": id, "site": "site B", "weekday": 3, "value": -1}
{"_id": id, "site": "site C", "weekday": 2, "value": 1}
{"_id": id, "site": "site A", "weekday": 1, "value": -1}
{"_id": id, "site": "site B", "weekday": 3, "value": 1}

我需要的是：

对于单个站点，比如说“站点A”，我需要每个工作日（总共7个）的字典列表，其中“值”的计数大于0，等于0，小于0。全部按工作日排序

因此，我的输出应该如下所示：

{"weekday": 1, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 2, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 3, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 4, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 5, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 6, "greaterCount": x, "lesserCount": y, "zeroCount": z}
{"weekday": 7, "greaterCount": x, "lesserCount": y, "zeroCount": z}

---

当然，greaterCount、lesserCount和zeroCount的值在不同的工作日会有所不同，我的示例输出中的每个字典中都有x、y和z，因为我很懒。

这里基本上要查找的是运算符。这是一个“三元”条件，其计算结果是从逻辑条件的

true/false

返回一个值

在这种情况下，每个“逻辑”测试都会查看当前的“值”字段，并确定测试的

true

位置，例如是返回正值还是返回

0

值：

db.collection.aggregate([ 
    { "$group": {
        "_id": "$weekday",
        "greaterCount": {
            "$sum": {
                "$cond": [
                    { "$gt": [ "$value", 0 ] },
                    1,
                    0
                ]
            }
        },
        "lesserCount": {
            "$sum": {
                "$cond": [
                    { "$lt": [ "$value", 0 ] },
                    1,
                    0
                ]
            }
        },
        "zeroCount": {
            "$sum": {
                "$cond": [
                    { "$eq": [ "$value", 0 ] },
                    1,
                    0
                ]
            }
        }

    }}
])

在样本上产生以下结果：

{ "_id" : 3, "greaterCount" : 2, "lesserCount" : 1, "zeroCount" : 0 }
{ "_id" : 2, "greaterCount" : 1, "lesserCount" : 0, "zeroCount" : 2 }
{ "_id" : 1, "greaterCount" : 1, "lesserCount" : 1, "zeroCount" : 0 }

---

这就是我错过的，$cond使它变得非常简单。尽管聚合查询中括号的数量仍然庞大。谢谢你的帮助！