db={"collection1":[{_id:"90b992b7-b85f-4e1c-9080-fc3e2dba0db0",pIds:["47c9124d-f027-4221-8d60-9f491993d923","08c89e83-ad62-443c-a731-3c6ccd9ca3af"]}],"collection2":[{_id:"47d44016-ff84-44dc-b650-abab36d5f661",userId:"47c9124d-f027-4221-8d60-9f491993d923",isOnline:true}]}

db.collection1.aggregate([{"$unwind":"$pIds"},{"$lookup":{"from":"collection2","localField":"pIds","foreignField":"userId","as":"docs"}},{"$group":{"_id":"$_id","pids":{"$push":{userId:"$pIds",isOnline:{"$ifNull":[{$first:"$docs.isOnline"},false]}}}}}])