寻找适用于 mongodb 4.2.x 的替代结果投影

Question

根据我的最后一个问题 ()，我构建了一个查询来检查多个可能重叠的日期范围内的文档数量。

该查询适用于 MongoDB 4.4，但我也需要运行它适用于 4.2。在 MongoDB 4.2 上，我收到以下错误：

Mongo Server error (MongoCommandException): Command failed with error 168 (InvalidPipelineOperator): 'Unrecognized expression '$first'' on server localhost:27017. 

The full response is:
{ 
    "ok" : 0.0, 
    "errmsg" : "Unrecognized expression '$first'", 
    "code" : 168.0, 
    "codeName" : "InvalidPipelineOperator"
}

您将如何编写聚合投影来实现相同的结果结构。这是带有数据设置的完整代码

db.createCollection("object_location_tracking");
db.getCollection("object_location_tracking").insertMany([
    {
        _id: "1",
        locationId: "locationA",
        objectId: "objectA",
        timestamp: ISODate("2020-01-01T00:00:00Z")
    },
    {
        _id: "2",
        locationId: "locationB",
        objectId: "objectA",
        timestamp: ISODate("2020-01-01T00:00:00Z")
    },
    {
        _id: "3",
        locationId: "locationA",
        objectId: "objectB",
        timestamp: ISODate("2019-01-01T00:00:00Z")
    },
    {
        _id: "4",
        locationId: "locationB",
        objectId: "objectB",
        timestamp: ISODate("2020-01-01T00:00:00Z")
    }
]);



db.getCollection("object_location_tracking").aggregate([
    {$facet: {
        "first_bucket_id": [
            {$match: {"objectId":"objectA",
                      "locationId":"locationA",
                      "timestamp": {$gte: new ISODate('2020-01-01'),
                                    $lt: new ISODate('2020-12-31')}
                     }},
            {$count: "N"}
        ],

        "second_bucket_id": [
            {$match: {"objectId":"objectA",
                      "locationId":"locationA",
                      "timestamp": {$gte: new ISODate('2020-01-01'),
                                    $lt: new ISODate('2022-12-31')}
                     }},
            {$count: "N"}
        ],

        "third_bucket_id": [
            {$match: {"objectId":"objectA",
                      "locationId":"locationB",
                      "timestamp": {$gte: new ISODate('2022-01-01'),
                                    $lt: new ISODate('2022-12-31')}
                     }},
            {$count: "N"}
        ]
    }},
    {
        $set: {
            first_bucket_id: { $first: "$first_bucket_id.N"},
            second_bucket_id: { $first: "$second_bucket_id.N"},
            third_bucket_id: { $first: "$third_bucket_id.N"}
        }
    }
    , {
        $project: {
            first_bucket_id: 1,
            second_bucket_id: 1,
            third_bucket_id: 1
            
        }
    }
]);

Answer 1

您正在使用 first array element，您可以看出这是为版本 4.4 添加的新运算符

幸运的是，这很容易克服，只需使用 $arrayElemAt，就像这样：

{
    $set: {
        first_bucket_id: {$arrayElemAt: ["$first_bucket_id.N", 0]},
        second_bucket_id: {$arrayElemAt: ["$second_bucket_id.N", 0]},
        third_bucket_id: {$arrayElemAt: ["$third_bucket_id.N", 0]}
    }
}

寻找适用于 mongodb 4.2.x 的替代结果投影

Looking for alternative result projection that works on mongodb 4.2.x

java

mongodb

aggregation-framework