Mongodb: Not expected result from aggregate ($lookup) - mongodb

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.

Answer 2

If You have multiple objectIds and need in one array Boxes:
Ex: "boxes": [
{
"_id": ObjectId("5ac6bb69f3c36e17f0d34bd2")
},
{
"_id": ObjectId("5ac6bb69f3c36e17f0d34bd3")
}
]
Then u need to do group
Query :
db.getCollection('business').aggregate([
{"$unwind": "$boxes"} ,
{$lookup:{
from: "boxes",
localField: "boxes._id",
foreignField: "_id",
as: "box"
}},
{
$group: {
_id: '$_id',
name: { $first: '$name' },
boxes: {
$push: {
_id: '$boxes._id',
name: '$boxes.name'
color: '$boxes.color'
}
}
}
}
])