I must utilize the MongoDB native driver to retrieve unique IDs sorted by a timestamp column

I am currently utilizing the nodejs mongodb native driver for developing a chat application. Within my database, I have a collection named dialog which contains fields for sessionId and dateCreated (timestamp). My objective is to retrieve a list of distinct sessionIds ordered by their latest (max) dateCreated timestamp, with a limit of 20 records.

Below is my current implementation:

db.collection('dialog').distinct('sessionId',
        function(err, users){
            if(users.length > 10){
                users = users.slice(0, 20);
            }
            console.log("users: " + users);


        });

Here are a couple sample documents from the collection:

{ 
  "sessionId" : 455206183, 
  "msg" : "hi what's up?", 
  "dateCreated" : ISODate("2015-02-05T20:17:55.418Z"), 
  "_id" : ObjectId("54d3cff30e3ddb6922fbc2bb") 
}, { 
  "sessionId" : 163220612, 
  "msg" : "yo", 
  "dateCreated" : ISODate("2015-02-05T20:18:00.434Z"), 
  "_id" : ObjectId("54d3cff80e3ddb6922fbc2bc") 
}
javascriptmongodb

Answer №1

In the feedback provided, it has been suggested that in order to obtain the desired results, an aggregate function may be necessary. The following code snippet could potentially help achieve this:

db.dialog.aggregate([
    { $sort: { "$dateCreated": -1, "$sessionId": 1 }},
    { $group: { _id: "$sessionId", dateCreated: {$first: "$dateCreated"} }},
    { $limit: 20 },
    { $project: { sessionId: 1, _id: 0 }}
]);

This script initially sorts the collection based on dateCreated and sessionId, then groups them by unique session IDs while selecting the latest $first date created for each group. Afterwards, it limits the output to 20 entries and presents an array of objects with only the sessionId values included.

Please bear in mind that I have not personally tested this code snippet, so feel free to provide feedback on its performance.

Answer №2

Following Olli K's advice really helped me find the right path. It turns out that sorting after grouping by works perfectly.

db.collection('dialog').aggregate([
        {$group: {_id:"$sessionId", dateCreated: {$first: "$dateCreated"} }},
        {$sort: {"dateCreated":-1}},
        {$limit: 20}],
function(error, users){
    //do something
});

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