uses for mongodb ObjectId creation time

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.

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);
   }
}

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/