how to get user object by userid from result of aggregation

Question

I'm trying to query mongoDB to fetch data aggregation. Here is my document:

Relations:

{
    "_id" : 1,
    "from" : "a",
    "to" : "b",
    "message" : "a to b",
    "createdAt" : ISODate("2015-06-06T16:42:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:42:32.789Z")
}
{
    "_id" : 2,
    "from" : "a",
    "to" : "c",
    "message" : "a to c",
    "createdAt" : ISODate("2015-06-06T16:43:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:43:32.789Z")
}
{
    "_id" : 3,
    "from" : "b",
    "to" : "c",
    "message" : "b to c",
    "createdAt" : ISODate("2015-06-06T16:44:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:44:32.789Z")
}
{
    "_id" : 4,
    "from" : "a",
    "to" : "c",
    "message" : "a to c2",
    "createdAt" : ISODate("2015-06-06T16:45:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:45:32.789Z")
}
{
    "_id" : 5,
    "from" : "b",
    "to" : "c",
    "message" : "b to c2",
    "createdAt" : ISODate("2015-06-06T16:46:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:46:32.789Z")
}

User:

{
    "_id" : 'a',
    "name" : "q",
    "createdAt" : ISODate("2015-06-14T17:20:27.288Z"),
    "updatedAt" : ISODate("2015-06-15T14:24:30.383Z")
}
{
    "_id" : 'b',
    "name" : "e",
    "createdAt" : ISODate("2015-06-14T17:20:29.288Z"),
    "updatedAt" : ISODate("2015-06-15T14:24:3.383Z")
}
{
    "_id" : 'c',
    "name" : "t",
    "createdAt" : ISODate("2015-06-14T17:20:28.288Z"),
    "updatedAt" : ISODate("2015-06-15T14:24:38.383Z")
}

I've tried this:

db.collection.aggregate([
    {
        "$sort": {
            "updatedAt": -1
        }
    },
    {
        "$group": {
            "_id": {
                "to": "$to",
                "from": "$from"                
            },
            "id": { "$first": "$_id" },
            "message": { "$first": "$message" },
            "createdAt": { "$first": "$createdAt" },
            "updatedAt": { "$first": "$updatedAt" }
        }
    },
    {
        "$project": {
            "_id" : 0,
            "id": 1,
            "from" : "$_id.from",
            "to": "$_id.to",
            "message": 1,
            "createdAt": 1,
            "updatedAt": 1
        }
    }
]);

I've got this:

{
    "_id" : 1,
    "from" : "a",
    "to" : "b",
    "message" : "a to b",
    "createdAt" : ISODate("2015-06-06T16:42:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:42:32.789Z")
}
{
    "_id" : 4,
    "from" : "a",
    "to" : "c",
    "message" : "a to c2",
    "createdAt" : ISODate("2015-06-06T16:45:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:45:32.789Z")
}
{
    "_id" : 5,
    "from" : "b",
    "to" : "c",
    "message" : "b to c2",
    "createdAt" : ISODate("2015-06-06T16:46:32.789Z"),
    "updatedAt" : ISODate("2015-06-06T16:46:32.789Z")
}

Now, i want to get one document with recent to-from combination. Example:

{
        "_id" : 1,
        "from" :{
            "_id" : 'a',
            "name" : "q",
            "createdAt" : ISODate("2015-06-14T17:20:27.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:30.383Z")
        },
        "to" : {
            "_id" : 'b',
            "name" : "e",
            "createdAt" : ISODate("2015-06-14T17:20:29.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:3.383Z")
        },
        "message" : "a to b",
        "createdAt" : ISODate("2015-06-06T16:42:32.789Z"),
        "updatedAt" : ISODate("2015-06-06T16:42:32.789Z")
}
{
        "_id" : 4,
        "from" :{
            "_id" : 'a',
            "name" : "q",
            "createdAt" : ISODate("2015-06-14T17:20:27.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:30.383Z")
        },
        "to" : {
            "_id" : 'c',
            "name" : "t",
            "createdAt" : ISODate("2015-06-14T17:20:28.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:38.383Z")
        },
        "message" : "a to c2",
        "createdAt" : ISODate("2015-06-06T16:45:32.789Z"),
        "updatedAt" : ISODate("2015-06-06T16:45:32.789Z")
}
{
        "_id" : 5,
        "from" : {
            "_id" : 'b',
            "name" : "e",
            "createdAt" : ISODate("2015-06-14T17:20:29.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:3.383Z")
        },
        "to" : {
            "_id" : 'c',
            "name" : "t",
            "createdAt" : ISODate("2015-06-14T17:20:28.288Z"),
            "updatedAt" : ISODate("2015-06-15T14:24:38.383Z")
        },
        "message" : "b to c2",
        "createdAt" : ISODate("2015-06-06T16:46:32.789Z"),
        "updatedAt" : ISODate("2015-06-06T16:46:32.789Z")
}

Any help with code is appreciated.

Answer 1

Since MongoDB doesn't support joins, you can merge the two collections into an object array by using the forEach() method of the aggregate() cursor to iterate the cursor and access the documents in both collections, as in the following example:

var pipeline = [
    {
        "$sort": {
            "updatedAt": -1
        }
    },
    {
        "$group": {
            "_id": {
                "to": "$to",
                "from": "$from"                
            },
            "id": { "$first": "$_id" },
            "message": { "$first": "$message" },
            "createdAt": { "$first": "$createdAt" },
            "updatedAt": { "$first": "$updatedAt" }
        }
    },
    {
        "$project": {
            "_id" : 0,
            "id": 1,
            "from" : "$_id.from",
            "to": "$_id.to",
            "message": 1,
            "createdAt": 1,
            "updatedAt": 1
        }
    }],
    cur = db.collection.aggregate(pipeline),
    result = [];

cur.forEach(function (doc){
    var toUser = db.user.findOne({"_id": doc.to});
    var fromUser = db.user.findOne({"_id": doc.from});
    doc.to = toUser;
    doc.from = fromUser;
    result.push(doc);
})

printjson(result);

Output:

[
        {
                "id" : 1,
                "message" : "a to b",
                "createdAt" : ISODate("2015-06-06T16:42:32.789Z"),
                "updatedAt" : ISODate("2015-06-06T16:42:32.789Z"),
                "from" : {
                        "_id" : "a",
                        "name" : "q",
                        "createdAt" : ISODate("2015-06-14T17:20:27.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:30.383Z")
                },
                "to" : {
                        "_id" : "b",
                        "name" : "e",
                        "createdAt" : ISODate("2015-06-14T17:20:29.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:00Z")
                }
        },
        {
                "id" : 4,
                "message" : "a to c2",
                "createdAt" : ISODate("2015-06-06T16:45:32.789Z"),
                "updatedAt" : ISODate("2015-06-06T16:45:32.789Z"),
                "from" : {
                        "_id" : "a",
                        "name" : "q",
                        "createdAt" : ISODate("2015-06-14T17:20:27.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:30.383Z")
                },
                "to" : {
                        "_id" : "c",
                        "name" : "t",
                        "createdAt" : ISODate("2015-06-14T17:20:28.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:38.383Z")
                }
        },
        {
                "id" : 5,
                "message" : "b to c2",
                "createdAt" : ISODate("2015-06-06T16:46:32.789Z"),
                "updatedAt" : ISODate("2015-06-06T16:46:32.789Z"),
                "from" : {
                        "_id" : "b",
                        "name" : "e",
                        "createdAt" : ISODate("2015-06-14T17:20:29.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:00Z")
                },
                "to" : {
                        "_id" : "c",
                        "name" : "t",
                        "createdAt" : ISODate("2015-06-14T17:20:28.288Z"),
                        "updatedAt" : ISODate("2015-06-15T14:24:38.383Z")
                }
        }
]