比较同一集合中 2 个文档的字段以产生差异

Question

我有一个系统 我希望生成集合中文档在特定时间戳发生的更改历史记录

假设我们在集合中有一个文档

[
  {
    "property1" : "A",
    "property2" : "B",
    "createdAt" : "2023-05-18T06:36:00.000+00:00"
  }
]

更改为：

[
  {
    "property1" : "A1",
    "property2" : "B",
    "createdAt" : "2023-05-18T07:36:00.000+00:00"
  }
]

然后对于createdAt值

"2023-05-18T07:36:00.000+00:00"

我希望产生像

这样的输出

[
  {
    "propertyChanged" : "property1",
    "oldValue": "A"
    "newValue": "A1"
  }
]

仍在尝试找出最适合此目的的聚合管道运营商

Answer 1

沿着这些思路怎么样：

1. 使用

   $match

   根据时间戳过滤文档

   db.collection.aggregate([
     {
       $match: {
         createdAt: "2023-05-18T07:36:00.000+00:00"             }
     },
   
      //2. Use lookup to perform a self-join on the collection to find the previous version of the document based on the createdAt field.
     {
       $lookup: {
         from: "collection",
         let: { createdAt: "$createdAt" },
         pipeline: [
           {
             $match: {
               $expr: {
                 $lt: ["$$createdAt", "$createdAt"]
               }
             }
           },
           { $sort: { createdAt: -1 } },
           { $limit: 1 }
         ],
         as: "previousVersion"
       }
     },
     //3. Use unwind to destructure the arrays from the last operation
     {
       $unwind: "$previousVersion"
     },
   
     //4. Finally you can use project to compare the fields
     {
       $project: {
         propertyChanged: {
           $setDifference: [
             { $objectToArray: "$previousVersion" },
             { $objectToArray: "$$ROOT" }
           ]
         }
       }
     },
     {
       $unwind: "$propertyChanged"
     },
     {
       $project: {
         propertyChanged: "$propertyChanged.k",
         oldValue: {
           $arrayElemAt: ["$previousVersion", "$propertyChanged.k"]
         },
         newValue: {
           $arrayElemAt: ["$$ROOT", "$propertyChanged.k"]
         }
       }
     }
   ]);