Join document inside nested array MongoDB - mongodb

Question

My first collection, called Scenes looks like this:
{
sceneId: "id1",
contentId: "contentId1",
comments: [{
comment: "abc",
userId: "test1"
},
{
comment: "def",
userId: "test2"
}]
},
{
sceneId: "id2",
contentId: "contentId2",
comments: [{
comment: "abc",
userId: "test1"
},
{
comment: "def",
userId: "test2"
}]
}
Any my second collection, called Userdatas, looks like this:
{
userId: "test1",
profilPicture: "def.jpg"
},
{
userId: "test2",
profilPicture: "abc.jpg"
}
And I want to join them in a way, that I get the following:
{
sceneId: "id1",
contentId: "contentId1",
comments: [{
comment: "abc",
userId: "test1",
profilPicture: "def.jpg"
},
{
comment: "def",
userId: "test2",
profilPicture: "abc.jpg"
}]
},
{
sceneId: "id2",
contentId: "contentId2",
comments: [{
comment: "abc",
userId: "test1",
profilPicture: "def.jpg"
},
{
comment: "def",
userId: "test2",
profilPicture: "abc.jpg"
}]
}
and I have no idea how to do this, all my previous attempts failed. Help is appreciated! The key problem is, that the $group operator will not show the contentId, when grouping via the sceneId

Answer 1

You can use this aggregation query:
First $unwind comments array to destructure and join by comments.userId.
Then $lookup that is like a JOIN, to merge userIds between collections generating a object called data.
Destructure $data to get values.
Add profilPicture to the object comments using $set.
$group by sceneId adding all comments into an array.
And use $projection to show only fields you want.
db.Scenes.aggregate([
{
"$unwind": "$comments"
},
{
"$lookup": {
"from": "Userdatas",
"localField": "comments.userId",
"foreignField": "userId",
"as": "data"
}
},
{
"$unwind": "$data"
},
{
"$set": {
"comments.profilPicture": "$data.profilPicture"
}
},
{
"$group": {
"_id": "$sceneId",
"comments": {
"$push": "$comments"
}
}
},
{
"$project": {
"_id": 0,
"sceneId": "$_id",
"comments": 1
}
}
])
Example here