$locf

运算符 $locf 在窗口查询的分区中向前传播最后一个观察到的非 null 值。 $locf运算符在填充时序数据或其他具有差距的数据集中缺少的数据点时特别有用。

Syntax

{
  $locf: {
    input: <expression>,
    sortBy: <document>
  }
}

参数

例子

请考虑商店集合中的这个示例文档。

{
    "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
    "name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
    "location": {
        "lat": -89.2384,
        "lon": -46.4012
    },
    "staff": {
        "totalStaff": {
            "fullTime": 8,
            "partTime": 20
        }
    },
    "sales": {
        "totalSales": 75670,
        "salesByCategory": [
            {
                "categoryName": "Wine Accessories",
                "totalSales": 34440
            },
            {
                "categoryName": "Bitters",
                "totalSales": 39496
            },
            {
                "categoryName": "Rum",
                "totalSales": 1734
            }
        ]
    },
    "promotionEvents": [
        {
            "eventName": "Unbeatable Bargain Bash",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 6,
                    "Day": 23
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 7,
                    "Day": 2
                }
            },
            "discounts": [
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 7
                },
                {
                    "categoryName": "Bitters",
                    "discountPercentage": 15
                },
                {
                    "categoryName": "Brandy",
                    "discountPercentage": 8
                },
                {
                    "categoryName": "Sports Drinks",
                    "discountPercentage": 22
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 19
                }
            ]
        },
        {
            "eventName": "Steal of a Deal Days",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 21
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 29
                }
            },
            "discounts": [
                {
                    "categoryName": "Organic Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "White Wine",
                    "discountPercentage": 20
                },
                {
                    "categoryName": "Sparkling Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 17
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 23
                }
            ]
        }
    ]
}

示例 1：用于 $locf 填充缺少的时序数据

若要跨“首次启动顾问”公司内的商店传播 lastUpdated 字段的最新非 null 值，请首先运行查询来对公司内的商店进行分区。 然后，使用 $lecf 运算符传播分区中所有存储的字段的最后一个非 null 值。

db.stores.aggregate([{
    "$match": {
        "company": {
            "$in": ["First Up Consultants"]
        }
    }
}, {
    "$setWindowFields": {
        "partitionBy": "$name",
        "sortBy": {
            "sales.revenue": 1
        },
        "output": {
            "lastUpdatedDate": {
                "$locf": {
                    "lastUpdated": 1
                }
            }
        }
    }
}, {
    "$project": {
        "company": 1,
        "name": 1,
        "lastObservedDiscount": 1
    }
}])

此查询返回的前三个结果为：

[
    {
        "_id": "0f4c48fe-c43b-4083-a856-afe6dd902077",
        "name": "First Up Consultants | Appliance Bargains - Feilmouth",
        "company": "First Up Consultants"
    },
    {
        "_id": "c4883253-7ccd-4054-a7e0-8aeb202307b5",
        "name": "First Up Consultants | Appliance Bargains - New Kari",
        "company": "First Up Consultants"
    },
    {
        "_id": "a159ff5c-6ec5-4ca8-9672-e8903a54dd90",
        "name": "First Up Consultants | Appliance Bargains - Schadenstad",
        "company": "First Up Consultants"
    }
]

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