[{"key":1,"b":[{"_id":"6258bdfe983a2d31e1cc6a4b","booking_room_id":"619395ba18984a0016caae6e","checkIn_date_time":"2022-04-16","checkOut_date_time":"2022-05-17"}],"r":[{"_id":"619395ba18984a0016caae6e"}]},{"key":2,"b":[{"_id":"6258bdfe983a2d31e1cc6a4b","booking_room_id":"619395ba18984a0016caae6e","checkIn_date_time":"2022-04-16","checkOut_date_time":"2022-05-17"}],"r":[{"_id":"619395ba18984a0016caae6e"},{"_id":"619395ba18984a0016caae6f"},{"_id":"619395ba18984a0016caae7a"}]}]

db.collection.aggregate([{$match:{$expr:{$ne:[{$filter:{input:"$r",as:"r",cond:{$not:{$in:["$$r._id","$b.booking_room_id"]}}}},[]]}}},{$set:{r:{$filter:{input:"$r",as:"r",cond:{$not:{$in:["$$r._id","$b.booking_room_id"]}}}}}}])