Merge two $lookup collections in MongoDB - mongodb

Question

Here i am try to get aggregated result from my challenge collection with challengeusers and challengeusers has the user_id and i used $lookup to join the users too.
When i use this query, i am getting following output.
"challenges": [
{
"_id": "5b7bf6fd87ec106308d7e3c1",
"start_date": "2018-08-09T12:40:21.470Z",
"end_date": "2018-08-05T12:40:21.470Z",
"challnegedusers": [
{
"chalenge_id": "5b7bf6fd87ec106308d7e3c1",
"user_id": "5b75623db457045e3bb12e0a",
"status": 1
},
{
"user_id": "5b75643c0a97791bcc9ed64c",
"status": 1
},
{
"user_id": "5b756144b457045e3bb12e08",
"status": 1
}
],
"users": [
{
"_id": "5b756144b457045e3bb12e08",
"first_name": "XYZ"
},
{
"_id": "5b75623db457045e3bb12e0a",
"first_name": "BAC"
},
{
"_id": "5b75643c0a97791bcc9ed64c",
"first_name": "YTA"
}
]
}
]
But i want the challengeusers and users to merge in a single object.
Most of all i want the status of challengeusers with user's info.
expected output:
"challenges": [
{
"_id": "5b7bf6fd87ec106308d7e3c1",
"start_date": "2018-08-09T12:40:21.470Z",
"end_date": "2018-08-05T12:40:21.470Z",
"challnegedusers": [
{
"user_id": "5b75623db457045e3bb12e0a",
"status": 1,
"first_name": "BAC"
},
{
"user_id": "5b75643c0a97791bcc9ed64c",
"status": 1,
"first_name": "YTA"
},
{
"user_id": "5b756144b457045e3bb12e08",
"status": 1,
"first_name": "XYZ"
}
]
}
]
MongoDB Aggregate Query that i am using.
let challenges = await ChallengeModel.aggregate([
{ $match: criteria },
{ $lookup: {
from: 'challengeusers',
localField: '_id',
foreignField: 'challenge_id',
as: 'challnegedusers'
} },
{ $lookup: {
from: 'appusers',
localField: 'challnegedusers.user_id',
foreignField: '_id',
as: 'users'
} },
{ $sort: {created_at: -1}}
]);

Answer 1

You can try below aggregation in mongodb 3.6
ChallengeModel.aggregate([
{ "$match": criteria },
{ "$lookup": {
"from": "challengeusers",
"let": { "challengeusersId": "$_id" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$challenge_id", "$$challengeusersId" ] } } },
{ "$lookup": {
"from": "appusers",
"let": { "user_id": "$user_id" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$_id", "$$user_id" ] } } },
],
"as": "user"
}},
{ "$unwind": "$user" },
{ "$addFields": { "first_name": "$user.first_name" }},
{ "$project": { "user": 0 }}
],
"as": "challnegedusers"
}}
])
And with your approach you can try this
ChallengeModel.aggregate([
{ "$match": criteria },
{ "$lookup": {
"from": "challengeusers",
"localField": "_id",
"foreignField": "challenge_id",
"as": "challnegedusers"
}},
{ "$unwind": "challnegedusers" },
{ "$lookup": {
"from": "appusers",
"localField": "challnegedusers.user_id",
"foreignField": "_id",
"as": "challnegedusers.user"
}},
{ "$unwind": "challnegedusers.user" },
{ "$addFields": { "challnegedusers.first_name": "$challnegedusers.user.first_name" }},
{ "$sort": { "created_at": -1 }},
{ "$group": {
"_id": "$_id",
"start_date": { "$first": "$start_date" }
"end_date": { "$first": "$end_date" },
"challnegedusers": { "$push": "$challnegedusers" }
}}
])