如何在Node.js中实现mongo ttl？

Question

我必须实现mongodb ttl才能从集合中删除文档。    我已将文档设置为60秒过期，但不会删除。    该文档没有被删除：

MongoClient.connect(url,function(err,db){
        if(err) throw err;
        var dbo = db.db("mydb");
        dbo.collection("users").drop();
        dbo.collection("users").createIndex({"created_at": 1},{expireAfterSeconds: 60});
        dbo.collection("users").insert({name: "why","created_at" : new Date()});
    })
> db.users.find()
{ "_id" : ObjectId("5ca042066a4e9f23f0324196"), "name" : "why", "created_at" : ISODate("2019-03-31T04:28:54.128Z") }

Query of getIndexes:  
> db.users.getIndexes()
[
        {
                "v" : 2,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "mydb.users"
        },
        {
                "v" : 2,
                "key" : {
                        "created_at" : 1
                },
                "name" : "created_at_1",
                "ns" : "mydb.users",
                "expireAfterSeconds" : 60
        }
]