MongoDB aggregate Query with select fields

Question

I want to make chat system, and also need get last message of user which aggigation. I also provide query with this but it only return userId of user. so please help me, thanks

Database:

/* 1 */
{
    "_id" : ObjectId("56937df0418a6afab248616d"),
    "to" : ObjectId("56728051d4b426be03de18f2"),
    "from" : ObjectId("568e402eaecfa53282f60d17"),
    "msg" : "Hello!",
    "cd" : ISODate("2016-01-11T10:03:28.139Z"),
    "type" : "other",
    "ir" : 0
}

/* 2 */
{
    "_id" : ObjectId("56937e01418a6afab248616e"),
    "to" : ObjectId("568e402eaecfa53282f60d17"),
    "from" : ObjectId("56728051d4b426be03de18f2"),
    "msg" : "Hi!",
    "cd" : ISODate("2016-01-11T10:03:45.588Z"),
    "type" : "other",
    "ir" : 0
}

/* 3 */
{
    "_id" : ObjectId("56937e45418a6afab248616f"),
    "to" : ObjectId("56728051d4b426be03de18f2"),
    "from" : ObjectId("568e402eaecfa53282f60d17"),
    "msg" : "Shu che ela!",
    "cd" : ISODate("2016-01-11T10:04:53.280Z"),
    "type" : "other",
    "ir" : 0
}

Query:

db.getCollection('chat_message').aggregate( [
        { 
            $match: {
                ir: 0, 
                $or : [
                    {"to" : ObjectId("56728051d4b426be03de18f2")}
                ]  
            } 
        },

        { $group: { _id: "$from" } },

    ])

I Run this query but not get result which i want

Require Out came:

/* 1 */
{
    "result" : [ 
        {
            "_id" : ObjectId("568e402eaecfa53282f60d17"),
            "msg" : "Shu che ela!"
        }
    ],
    "ok" : 1.0000000000000000
}

Neo-coder · Accepted Answer · 2016-01-11 13:00:43Z

3

You are right track but you missing some things about $mongo object Id

In MongoDB, documents stored in a collection require a unique _id field that acts as a primary key.

so when you run your aggregation query $group creates _id for from key but in your document structure looks like from having two documents with same ObjectId since the result return first matching criteria. This return only "msg" : "Hello!", or "msg" : "Shu che ela!" which documents inserted first return first.

so should changed your aggreation like this {"$group":{"_id":"$_id","msg":{"$first":"$msg"}}} you will get both documents.

answered Jan 11, 2016 at 13:00

Neo-coder

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1 Comment

Hardik Ranpariya Over a year ago

db.getCollection('chat_message').aggregate( [ { $match: { ir: 0, $or : [ {"to" : ObjectId("56728051d4b426be03de18f2")} ] } }, {"$group":{"_id":"$from","msg":{"$last":"$msg"}}} ]) I put this query and i got last message as par i want. thanks.

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