Mongodb: Not expected result from aggregate ($lookup)

Question

I´m looking for some help.

I have a collection called 'business' where I have objects like this:

[{
    "_id": ObjectId("5aefa97166763d28fc984c7a"),
    "name": "Business 1",
    "boxes": [
        {
            "_id": ObjectId("5ac6bb69f3c36e17f0d34bd2")
        }
    ]
}]

There is another collection called boxes which has objects like:

[{
    _id: ObjectId("5ac6bb69f3c36e17f0d34bd2"),
    name:"Box1",
    color:"blue"
}]

The idea here is that there are businesses that own boxes and I want to keep both collection separated.

That said, I would like to retrieve this result:

[{
    "_id": ObjectId("5aefa97166763d28fc984c7a"),
    "name": "Business 1",
    "boxes": [{
        _id: ObjectId("5ac6bb69f3c36e17f0d34bd2"),
        name:"Box1",
        color:"blue"
    }]
}]

But I am getting this result:

[{
    "_id": ObjectId("5aefa97166763d28fc984c7a"),
    "name": "Business 1",
    "boxes": [
        {
            "_id": ObjectId("5ac6bb69f3c36e17f0d34bd2")
        }
    ]
}]

Using $lookup as you can see below:

db.db('database').collection("business").aggregate({
    $lookup:{
        from: "boxes",
        localField: "_id",
        foreignField: "_id",
        as: "box"
    },
    "$unwind": "$boxes" 
    }).toArray(function(err, result) {
        if (err) throw err; 
        res.send(result);
        db.close();
        res.end();
    });

What am I doing wrong?

Thank you all!

Answer 1

This should help

db.business.aggregate([{$lookup:{from: "boxes", localField: "boxes._id", foreignField: "_id", as: "box" }},{"$project":{"_id":1,"name":1,"boxes":"$box"}}])

The lookup creates an array "box" which has all matching documents from the boxes collection.The next stage in the pipeline, $project , selects _id and name from the new document and renames the box array to boxes.