[{"key":1},{"key":2},{"key":3},{"key":4},{"key":5},{"key":6}]

db.collection.aggregate([{$facet:{data:[],sample:[{$sample:{size:5}}]}},{$project:{data:{$cond:[{$gt:[{$size:"$data"},5]},"$sample","$data"]}}},{$unwind:"$data"},{$replaceWith:"$data"}])