MongoDB Aggregate Pipeline query - node.js

Question

I'm using mongoDB 3.6 on node.js 8.11.1 and working with MongoDB Node.js Driver.
I have two collections, 'group' and 'user':
group:
[
{
"_id":1,
"groupName":"group1",
"users":[
{
"userId":1,
"isAdmin":"false"
},
{
"userId":2,
"isAdmin":"true"
}
]
},
{
"_id":2,
"groupName":"group2",
"users":[
{
"userId":2,
"isAdmin":"false"
},
{
"userId":3,
"isAdmin":"true"
}
]
}
]
user:
[
{
"_id":1,
"username":"user1",
"firstname":"a",
"lastname":"aa",
"mobileNo":"+1111111"
},
{
"_id":2,
"username":"user2",
"firstname":"b",
"lastname":"bb",
"mobileNo":"+2222222"
},
{
"_id":3,
"username":"user3",
"firstname":"c",
"lastname":"cc",
"mobileNo":"+3333333"
}
]
I need an aggregate to return something like this:
[
{
"_id":1,
"groupName":"group1",
"members":[
{
"isAdmin":"false",
"username":"user1",
"firstname":"a",
"lastname":"aa"
},
{
"isAdmin":"true",
"username":"user2",
"firstname":"b",
"lastname":"bb"
}
]
},
{
"_id":2,
"groupName":"group2",
"members":[
{
"isAdmin":"false",
"username":"user2",
"firstname":"b",
"lastname":"bb"
},
{
"isAdmin":"true",
"username":"user3",
"firstname":"c",
"lastname":"cc"
}
]
}
]
At "members" in result, "isAdmin" return from "users" at group collection and "username", "firstname" and "lastname" came from user collection
Many thanks,
Milad.

Answer 1

You can try below aggregation from mongodb 3.6 and above
db.group.aggregate([
{ "$unwind": "$users" },
{ "$lookup": {
"from": Users.collection.name,
"let": { "userId": "$users.userId", "isAdmin": "$users.isAdmin" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$_id", "$$userId" ] } } },
{ "$project": { "isAdmin": "$$isAdmin", "username": 1, "firstName": 1, "lastName": 1 }}
],
"as": "members"
}},
{ "$unwind": "$members" },
{ "$group": {
"_id": "$_id",
"members": { "$push": "$members" },
"groupName": { "$first": "$groupName" }
}}
])