Aggregate multiple collections based on student Id - arrays

Question

I am trying to aggregate 2 collections in MongoDB based on a student's ID. One collection consists of student personal information, another one consists of the students logs. The issue is that the data is in array which is why I think my aggregation is not working. Any help will be appreciated.
student collection
{
"_id" : ObjectId("(Object ID here"),
"data" : [
{
"name" : "John",
"id" : 1
},
{
"name" : "Sandy",
"id" : 2
}
]
}
logs collection
{
"_id" : ObjectId("(Object ID here"),
"logs" : [
{
"studentId" : 1,
"activity" : "11112,334,123"
},
{
"studentId" : 2,
"activity" : "11112,334,123"
}
]
}
Here is what I have tried:
dbo.collection("student").aggregate([
{ "$lookup": {
"localField": "data.id",
"from": "logs",
"foreignField": "logs.studentId",
"as": "studentInfo"
}
}]).toArray(function(err, results) {
console.log(results);
});
Expected result:
studentinfo: {
id: 1,
name: "John",
activity" : "11112,334,123"
}

Answer 1

You can use below aggregation with mongodb 3.6
So basically your foreign field is an array you need to use $lookup with the pipeline to $unwind the foreign array inside the $lookup pipeline and to match the corresponding ids.
db.students.aggregate([
{ "$lookup": {
"from": "logs",
"let": { "dataId": "$data.id" },
"pipeline": [
{ "$unwind": "$logs" },
{ "$match": { "$expr": { "$in": ["$logs.studentId", "$$dataId"] }}},
{ "$replaceRoot": { "newRoot": "$logs" }}
],
"as": "students"
}}
])
or use this to merge both the arrays
db.students.aggregate([
{ "$lookup": {
"from": "logs",
"let": { "dataId": "$data.id" },
"pipeline": [
{ "$unwind": "$logs" },
{ "$match": { "$expr": { "$in": ["$logs.studentId", "$$dataId"] }}},
{ "$replaceRoot": { "newRoot": "$logs" }}
],
"as": "students"
}},
{ "$project": {
"students": {
"$map": {
"input": "$students",
"in": {
"studentId": "$$this.studentId",
"activity": "$$this.activity",
"name": { "$arrayElemAt": ["$data.name", { "$indexOfArray": ["$data.id", "$$this.studentId"] }]}
}
}
}
}}
])
Output
[
{
"students": [
{
"activity": "11112,334,123",
"name": "John",
"studentId": 1
},
{
"activity": "11112,334,123",
"name": "Sandy",
"studentId": 2
}
]
}
]