Pymongo 和聚合框架查询

Question

我尝试在 pymongo 中编写聚合查询，但我自己做不到，我尝试了文档，尝试了 google，尝试了 Stack Overflow，但我找不到答案。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}

而我需要的是：

对于单个站点，假设 "site A"，我需要每个工作日的词典列表（总共 7 个），"values" 的计数大于 0、等于 0 且小于 0。所有按工作日排序。

所以我的输出应该是这样的：

{"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。

Answer 1

您在这里寻找的基本上是 $cond 运算符。这是一个 "ternary" 条件，其计算结果为 return 来自逻辑条件 true/false 的值。

在这种情况下，每个 "logical" 测试都会查看当前的 "value" 字段并确定 true 到 $gt whether to return a positive value to $sum 或 0 等测试的位置值改为：

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
                ]
            }
        }

    }}
])

在样品上产生：

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

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