Mongo Aggregation $subtract ：对于两个非空日期总是给出空结果答案

【问题标题】：Mongo Aggregation $subtract : Always gives null result for two non empty datesMongo Aggregation $subtract ：对于两个非空日期总是给出空结果
【发布时间】：2022-01-22 21:46:33
【问题描述】：

我想获取一系列员工工作纪念日的列表。加入日期在员工记录中可用。在此查询中，首先计算今年的 aniversaryDate，然后尝试找出今天日期和周年日期之间的差异。但是$subtract 对所有记录集都返回 false。

db.employees.aggregate([{
    "$match": { "deleted": false }
},
{
    "$project": {
        "id": 1, "firstName": 1, "lastName": 1, "empId": 1, "joiningDate": 1,
        "aniversaryDate":
            {
                "$dateFromParts":
                    {
                        "year": { "$year": "$$NOW" },
                        "day": { "$dayOfMonth": "$joiningDate" },
                        "month": { "$month": "$joiningDate" }
                    }
            },
        "dateDifference":
            { "$subtract": ["$aniversaryDate", "$$NOW"] }


    }
}, { "$sort": { "dateDifference": 1 } }]);

结果给出了这样的对象数组：

{
    "_id" : ObjectId("61a4bf6b5362437b668db2b5"),
    "empId" : "1",
    "firstName" : "Olive",
    "lastName" : "Gunter",
    "joiningDate" : ISODate("2021-10-03T16:16:58.997+05:30"),
    "aniversaryDate" : ISODate("2021-10-03T05:30:00.000+05:30"),
    "dayssince" : null,
    "dateDifference" : null
}

【问题讨论】：

标签： mongodb aggregation-framework nosql-aggregation

【解决方案1】：

您需要另一个$project 阶段来获得aniversaryDate，它是从前一个$project 阶段生成的。

db.collection.aggregate([
  {
    "$match": {
      "deleted": false
    }
  },
  {
    "$project": {
      "id": 1,
      "firstName": 1,
      "lastName": 1,
      "empId": 1,
      "joiningDate": 1,
      "aniversaryDate": {
        "$dateFromParts": {
          "year": {
            "$year": "$$NOW"
          },
          "day": {
            "$dayOfMonth": "$joiningDate"
          },
          "month": {
            "$month": "$joiningDate"
          }
        }
      },
    }
  },
  {
    $project: {
      "dateDifference": {
        "$subtract": [
          "$aniversaryDate",
          "$$NOW"
        ]
      }
    }
  },
  {
    "$sort": {
      "dateDifference": 1
    }
  }
])

Sample Mongo Playground

如果您使用的是 MongoDB 版本 5，则可以替换为 $dateDiff。

{
  $project: {
    "dateDifference": {
      "$dateDiff": {
        "startDate": "$aniversaryDate",
        "endDate": "$$NOW",
        "unit": "day"
      }
    }
  }
}

Sample Mongo Playground (with $dateDiff)

【讨论】：

我无法更新 mongo 版本，所以我将使用 $subtract