$lookup and push new data - mongodb

Question

Hi i'm having a discussion object collection and a user deatils collection.
Discussion collection stores the participants username in an array of strings.
Discussion collection is as follows:
[{ "_id": "5a4dbdaab46b426863e7ead3",
"topic": "test",
"topicDesc": "test123",
"createdOn": "2018-01-04T05:37:46.088Z",
"participants": ["akhil","ben"] //[usernames]
}]
The user details collection is as follows:
[{
"_id": "59e6d6ba02e11e1814481022",
"username": "ben",
"name": "Ben S",
"email": "qwerty#123.com",
},{
"_id": "5a0431b1d6fab00cdf484677",
"username": "akhil",
"name": "Akhil Clement",
"email": "qwerty#123.com",
}]
and result JSON be like
[{ "_id": "5a4dbdaab46b426863e7ead3",
"topic": "test",
"topicDesc": "test123",
"createdOn": "2018-01-04T05:37:46.088Z",
"participants": ["akhil","ben"] //[usernames]
"participantDetails": [{
"_id": "59e6d6ba02e11e1814481022",
"username": "ben",
"name": "Ben S",
"email": "qwerty#123.com",
},{
"_id": "5a0431b1d6fab00cdf484677",
"username": "akhil",
"name": "Akhil Clement",
"email": "qwerty#123.com",
}]
}]

Answer 1

you need to $lookup with user collection and $group
db.dis.aggregate(
[
{$unwind : "$participants"},
{$lookup : {from : "us", localField : "participants", foreignField : "username", as : "userData"}},
{$group : {_id : {
_id : "$_id", topic : "$topic", topicDesc : "$topicDesc", createdOn : "$createdOn"
},
participants : {$push : "$participants" } ,
participantDetails : {$push : {$arrayElemAt : ["$userData", 0]}}}
},
{$project : {
_id : "$_id._id",
topic : "$_id.topic",
topicDesc : "$_id.topicDesc",
createdOn : "$_id.createdOn",
participants : 1 ,
participantDetails : 1
}}
]
).pretty()
result
{
"participants" : [
"akhil",
"ben"
],
"participantDetails" : [
{
"_id" : "59e6d6ba02e11e1814481020",
"username" : "akhil",
"name" : "Akhil Clement",
"email" : "qwerty#123.com"
},
{
"_id" : "59e6d6ba02e11e1814481021",
"username" : "ben",
"name" : "Ben S",
"email" : "qwerty#123.com"
}
],
"_id" : "5a4dbdaab46b426863e7ead3",
"topic" : "test",
"topicDesc" : "test123",
"createdOn" : "2018-01-04T05:37:46.088Z"
}
EDIT
change $push to $addToSet to avoid duplicates