db={"user":[{"_id":ObjectId("5f867e0d126ddbde24d6ee73"),"name":"Jaskson","age":"24"}],"Polls":[{"_id":ObjectId("5f87d988ddae726a3dbe5459"),"name":"RedVsWhite","total_votes":"0"},{"_id":ObjectId("5f51408ffc1d0437fa31d6f7"),"name":"ApplesVsOrange","total_votes":"0"}],"VotesRecords":[{"_id":ObjectId("5f864addddae726a3dbe53de"),"voter_id":ObjectId("5f867e0d126ddbde24d6ee73"),"poll_id":ObjectId("5f87d988ddae726a3dbe5459"),"voter_selection":"red"},{"_id":ObjectId("5f864addddae726a3dbe53df"),"voter_id":ObjectId("5f867e0d126ddbde24d6ee74"),"poll_id":ObjectId("5f87d988ddae726a3dbe5459"),"voter_selection":"red"}]}

db.Polls.aggregate([{$group:{_id:null,root:{$push:"$$ROOT"}}},{$lookup:{from:"VotesRecords",as:"VotesRecords",pipeline:[{$match:{voter_id:ObjectId("5f867e0d126ddbde24d6ee73")}}]}},{"$unwind":{"path":"$VotesRecords","preserveNullAndEmptyArrays":true}},{$project:{root:{$map:{input:"$root",in:{$mergeObjects:["$$this",{$cond:[{$eq:["$$this._id","$VotesRecords.poll_id"]},{voter_selection:"$VotesRecords.voter_selection"},{}]}]}}}}},{$unwind:"$root"},{$replaceWith:"$root"}])