db={"posts":[{"type":"event","_id":"63241dffb0f6770c23663230","user_id":"63241dffb0f6770c23663230","post_id":"63241dffb0f6770c23663230","likes":50},{"type":"poll","_id":"63241dffb0f6770c23663231","user_id":"63241dffb0f6770c23663230","post_id":"63241dffb0f6770c23663230","likes":50}],"events":[{"date":"2022-09-16T07:07:18.242+00:00","_id":"63241dffb0f6770c23663230","user_id":"63241dffb0f6770c23663230","venue":"Some Place","lat":"null","long":"null",}],"poll":[{"created_date":"2022-09-16T07:07:18.242+00:00","_id":"63241dffb0f6770c23663230","user_id":"63241dffb0f6770c23663230","question":"Question??????","poll_opt1":"Yes","poll_opt2":"No","poll_opt1_count":"5","poll_opt2_count":"2"}]}

db.posts.aggregate([{"$facet":{"eventPosts":[{"$match":{type:"event"},},{"$lookup":{"from":"events","localField":"post_id","foreignField":"_id","as":"post_id"}}],"pollPosts":[{"$match":{type:"poll"},},{"$lookup":{"from":"poll","localField":"post_id","foreignField":"_id","as":"post_id"}}]}},{"$addFields":{"doc":{"$concatArrays":["$pollPosts","$eventPosts"]}}},{"$unwind":"$doc"},{"$replaceRoot":{"newRoot":"$doc"}},{"$addFields":{"post_id":{"$cond":{"if":{"$eq":[{"$size":"$post_id"},0]},"then":{},"else":{"$arrayElemAt":["$post_id",0]}}}}}])