uses for mongodb ObjectId creation time

I suppose since MongoDB ObjectId contain a timestamp, you can sort by 'created date' if you will sort by objectId:

items.find.sort( [['_id', -1]] ) // get all items desc by created date.

And if you want last 30 created items you can use following query:

items.find.sort( [['_id', -1]] ).limit(30) // get last 30 createad items 

I am actualy not sure,i just suppose that ordering by _id should work as described above. I'll create some tests later.

Update:

Yes it is so. If you order by _id you will automatically order by _id created date. I've done small test in c#, mb someone interest in it:

  public class Item  {    [BsonId]    public ObjectId Id { get; set; }    public DateTime CreatedDate { get; set; }    public int Index { get; set; }  } [TestMethod] public void IdSortingTest() {   var server = MongoServer.Create("mongodb://localhost:27020");   var database = server.GetDatabase("tesdb");   var collection = database.GetCollection("idSortTest");   collection.RemoveAll();   for (int i = 0; i <= 500; i++)   {     collection.Insert(new Item() {              Id = ObjectId.GenerateNewId(),              CreatedDate = DateTime.Now,              Index = i });   }   var cursor = collection.FindAllAs<Item>();   cursor.SetSortOrder(SortBy.Descending("_id"));   var itemsOrderedById = cursor.ToList();   var cursor2 = collection.FindAllAs<Item>();   cursor2.SetSortOrder(SortBy.Descending("CreatedDate"));   var itemsOrderedCreatedDate = cursor.ToList();   for (int i = 0; i <= 500; i++)   {     Assert.AreEqual(itemsOrderedById[i].Index, itemsOrderedCreatedDate[i].Index);   }}

Yes, you can use the generation_time of BSON ObjectId for the purposes you want. So,

db.collection.find().sort({ _id : -1 }).limit(10)

will return the last 10 created items. However, since the embedded timestamps have a one second precision, multiple items within any second are stored in the order of their creation.

The code to convert a DateTime to its corresponding timestamp with the c# driver is as follows:

    public static ObjectId ToObjectId(this DateTime dateTime)    {        var timestamp = (int)(dateTime - BsonConstants.UnixEpoch).TotalSeconds;        return new ObjectId(timestamp, 0, 0, 0);    }

More info here: http://www.danharman.net/2011/10/26/mongodb-ninjitsu-using-objectid-as-a-timestamp/