[{"key":"4d"},{"key":"5d"},{"key":"6d"},{"key":"6d"}]

db.collection.aggregate([{$limit:1},{$project:{_id:1,list:["5d","5d","6d","7d"]}},{"$unwind":"$list"},{"$lookup":{"from":"collection","localField":"list","foreignField":"key","as":"match"}},{$project:{match:{$cond:[{$eq:["$match",[]]},[{_id:null,"key":"$list"}],"$match"]}}},{"$replaceWith":{$first:"$match"}}])