如何根据日期删除MongoDb中的文件

Question

So what I want to do is group all documents having same hash whose count is more than 1 and only keep the oldest record according to startDate

My db structure is as follows:

[{
  "_id": "82bacef1915f4a75e6a18406",
  "Hash": "cdb3d507734383260b1d26bd3edcdfac",
  "duration": 12,
  "price": 999,
"purchaseType": "Complementary",

  "startDate": {
    "$date": {
      "$numberLong": "1656409841000"
    }
  },
  "endDate": {
    "$date": {
      "$numberLong": "1687859441000"
    }
  }
 
}]

I was using this query which I created

db.Mydb.aggregate([
{
        "$group": {
         _id: {hash: "$Hash"},
         dups: { $addToSet: "$_id" } ,
         count: { $sum : 1 }
     }
 },{"$sort":{startDate:-1}},
 {
     "$match": {
                  count: { "$gt": 1 }
              }
 }
]).forEach(function(doc) {
   doc.dups.shift();
   db.Mydb.deleteMany({
       _id: {$in: doc.dups}
   });
})

this gives a result like this:

{ _id: { hash: '1c01ef475d072f207c4485d0a6448334' },
  dups: 
   [ '6307501ca03c94389f09b782',
     '6307501ca03c94389f09b783',
     '62bacef1915f4a75e6a18l06' ],
  count: 3 }

The problem with this is that the _id's in dups array are random everytime I run this query ie not sorted according to startDate field. What can be done here? Any help is appreciated. Thanks!

Answer 1

After $group stage, startDate field will not pre present in the results, so you can not sort based on that field. So, as stated in the comments, you should put $sort stage first in the Aggregation pipeline.

db.Mydb.aggregate([
  { 
    "$sort": { startDate: -1} 
  },
  {
    "$group": {
      _id: {hash: "$Hash"},
      dups: { $addToSet: "$_id" } ,
      count: { $sum : 1 }
  },
  {
    "$match": { count: { "$gt": 1 }
  }
]

Answer 2

Got the solution. I was using $addToSet in the group pipeline stage which does not allow duplicate values. Instead, I used $push which allows duplicate elements in the array or set.