Mongoose - find last message from each user

Question

I'm working on message system and I need to get last message from each user who sent message to logged user. I have this structure in mongoDB:

[{
    "_id": "551bd621bb5895e4109bc3ce",
    "from": "admin",
    "to": "user1",
    "message": "message1",
    "created": "2015-04-01T11:27:29.671Z"
}, {
    "_id": "551bd9acf26208ac1d9b831d",
    "from": "user1",
    "to": "admin",
    "message": "message2",
    "created": "2015-04-01T11:42:36.936Z"
}, {
    "_id": "551bdd6d849d53001dd8a64a",
    "from": "user1",
    "to": "user2",
    "message": "message3",
    "created": "2015-04-01T11:58:37.858Z"
}, {
    "_id": "551bdd99849d53001dd8a64b",
    "from": "user2",
    "to": "admin",
    "__v": 0,
    "message": "message4",
    "created": "2015-04-01T11:59:21.005Z"
}, {
    "_id": "551bdda1849d53001dd8a64c",
    "from": "user1",
    "to": "admin",
    "__v": 0,
    "message": "message5",
    "created": "2015-04-01T11:59:29.971Z"
}]

I need to get fields from, message and created from last message of each user who sent message to logged user. I tried use distinct but it returns only one field. I have this:

Message.find({
        to: req.user.username
    })
    .select('message created')
    .sort('-created')
    .exec(function (err, messages) {
        if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.json(messages)
        }
    });

but it returns all users who sent messages to logged user and I need to have only unique users and their last message. Is there some way how to do it with mongoose?

Answer 1

Use aggregation framework where your pipeline stages have $match, $sort, $group and $project expressions:

Message.aggregate(
    [
        // Matching pipeline, similar to find
        { 
            "$match": { 
                "to": req.user.username
            }
        },
        // Sorting pipeline
        { 
            "$sort": { 
                "created": -1 
            } 
        },
        // Grouping pipeline
        {
            "$group": {
                "_id": "$from",
                "message": {
                    "$first": "$message" 
                },
                "created": {
                    "$first": "$created" 
                }
            }
        },
        // Project pipeline, similar to select
        {
             "$project": { 
                "_id": 0,
                "from": "$_id",
                "message": 1,
                "created": 1
            }
        }
    ],
    function(err, messages) {
       // Result is an array of documents
       if (err) {
            return res.status(400).send({
                message: getErrorMessage(err)
            });
        } else {
            res.json(messages)
        }
    }
);

If req.user.username = "admin", with your sample collection then the result is:

{
    "result" : [ 
        {
            "message" : "message4",
            "created" : "2015-04-01T11:59:21.005Z",
            "from" : "user2"
        }, 
        {
            "message" : "message5",
            "created" : "2015-04-01T11:59:29.971Z",
            "from" : "user1"
        }
    ],
    "ok" : 1
}