Mongodb aggregation $in field of array element

Question

document structure

session document:

{
    "_id" : ObjectId("5648b811a16bc2e0355b8289"),
    "title" : "test",
    "messages" : [
        {
            "authorId" : ObjectId("5648b89f2fc311bc39073993"), // <-- the users id
            "created" : ISODate("2015-11-15T16:51:29.652Z"),
            "message" : "<p>test</p>"
        }
    ]
}

user document:

"_id" : ObjectId("5648b89f2fc311bc39073993"),
"pic" : "karl.png",
"profile" : {
    "name" : "Karl Morrison"
}

queries

first query:

var id = '5648b811a16bc2e0355b8289'; // <-- id for the session document
var session = // <-- returns the session document
    yield sessions.findOne({
        _id: id,
    });

second query (this is where I am stuck):

var sessionUsers =
        yield new Promise(function (resolve, reject) {
            users.col.aggregate([
                    {
                        $match: {
                            '_id': {
                                '$in': session.messages.authorId // <--- here I need help! Need all of the authorId's in the session document from the previous query result.
                            }
                        }
                                },
                    {
                        $project: {
                            _id: 1,
                            pic: 1,
                            name: '$profile.name'
                        }
                    }
                ],
                function (err, res) {
                    if (err === null)
                        resolve(res);
                    reject(err);
                });
        });

question

So I am trying to get all of the authorId's into an array and feed that array into the $in of the second query, which should return an array full of elements with the fields of _id, pic and name.

So sessionUsers variable should look something like this:

[{
    "_id" : ObjectId("5648b89f2fc311bc39073993"),
    "pic" : "karl.png",
    "name" : "Karl Morrison"
},...]

Thomas Bormans · Accepted Answer · 2015-11-15 17:43:22Z

1

I am not sure if I understand the problem correctly, but I think you are trying to send an array of objects instead of an array of _id's behind your $match. You will need to get all _id's in a simple array and use the result. You can use lodash. Here are the docs for more information.

The code below "plucks" all _id's from your session data.

var ids = _.pluck(session.messages, 'author‌Id');

Next, change the $match part to this:

{
    $match: {
        '_id': {
            '$in': ids
        }
    }
}

edited Nov 15, 2015 at 17:43

answered Nov 15, 2015 at 17:28

Thomas Bormans

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7 Comments

basickarl Over a year ago

You understand correctly, pluck function seems nice I'll look into it!

basickarl Over a year ago

Hmm the following var ids = _.pluck(session.messages, 'authorId'); echos with console.log(JSON.stringify(ids)); the following: [null]

Thomas Bormans Over a year ago

@KarlMorrison Strange. Have you checked session before using pluck?

basickarl Over a year ago

Indeed I have! console.log(JSON.stringify(session)) echos as it should:

{"_id":"5648c283026b46b03f5ca1df","title":"test","messages":[{"authorId":"5648c279026b46b03f5ca1d9","authorName":"Karl","created":"2015-11-15T17:36:03.439Z","message":"<p>test</p>"}]}

Thomas Bormans Over a year ago

My bad! Use _.pluck(session.messages, 'author‌Id'); instead :)

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