我们如何在 mongodb 中找到每个组的最近记录？

Question

I want to find out most recent record from each group. My collection:-

{ "_id" : ObjectId("1"), "loginID" : Ram.1234@gmail.com , "name" : Ram, "date" : ISODate("2017-02-01T00:00:00Z"),Site:blog.com,opertion:modified }
{ "_id" : ObjectId("2"), "loginID" : Arjun34@gmail.com, "name" : Arjun, "date" : ISODate("2017-02-01T00:00:00Z").site:ketanblog.com,opertion:updated }
{ "_id" : ObjectId("3"), "loginID" :Ram.1234@gmail.com , "name" : Ram, "date" : ISODate("2017-02-03T00:00:00Z"),Site:blogtec.com,opertion:modified }
{ "_id" : ObjectId("4"), "loginID" : Arjun34@gmail.com, "name" : Arjun, "date" : ISODate("2017-02-08T00:00:00Z"),Site:blogpec.com,opertion:updated }
{ "_id" : ObjectId("5"), "loginID" : Arjun34@gmail.com, "name" : Arjun, "date" : ISODate("2017-02-07T00:00:00Z") ,Site:blog.com,opertion:read}
{ "_id" : ObjectId("6"), "loginID" : shyam78@gmail.com, "name" : Shyam, "date" : ISODate("2017-02-09T00:00:00Z"),site:ketanblog.com,opertion:deleted }
{ "_id" : ObjectId("7"), "loginID" : shyam78@gmail.com, "name" : Shyam, "date" : ISODate("2017-02-03T00:00:00Z") ,site:ketanblog.com,opertion:updated}
{ "_id" : ObjectId("8"), "loginID" : Arjun34@gmail.com, "name" : Arjun, "date" : ISODate("2017-02-03T00:00:00Z"),Site:blogtrt.com,opertion:read  }

My Expected output:-

{ "_id" : ObjectId("3"), "loginID" : Ram.1234@gmail.com, "name" : Ram, "date" : ISODate("2017-02-03T00:00:00Z"),Site:blogtec.com,opertion:modified  }
{ "_id" : ObjectId("4"), "loginID" : Arjun34@gmail.com, "name" : Arjun, "date" : ISODate("2017-02-08T00:00:00Z"),Site:blogpec.com,opertion:updated  }
{ "_id" : ObjectId("6"), "loginID" : shyam78@gmail.com, "name" : Shyam, "date" : ISODate("2017-02-09T00:00:00Z"),Site:ketanblog.com,opertion:deleted  }

Answer 1

1. Sort by date to keep the latest data for each group at the top
2. Group by login id, get the first record for each group which is latest. It discards other records in each group

db.coll.aggregate([
{
  $sort : {"$date": - 1}
}, 
{
 $group: {
   _id : "$loginID", 
  data : { 
    $first : "$$ROOT" 
  } 
 } 
}
])

Answer 2

You can try,

• $sort by date descending order
• $group by loginID and add first recent document in recent
• $replaceRoot to replace recent into new root
• $sort new grouped documents

db.collection.aggregate([
  { $sort: { date: -1 } },
  {
    $group: {
      _id: "$loginID",
      recent: { $first: "$$ROOT" }
    }
  },
  { $replaceRoot: { newRoot: "$recent" } },
  { $sort: { date: -1 } }
])

Playground