用于mongodb ObjectId创建时间 [英] uses for mongodb ObjectId creation time

问题描述

用作mongodb文档中的默认键的ObjectId具有嵌入的时间戳(调用objectid.generation_time返回一个datetime对象).因此，可以使用此生成时间来代替保留单独的创建时间戳吗?您将如何使用此嵌入式时间戳按创建时间排序或有效查询最后N个项目?

The ObjectId used as the default key in mongodb documents has embedded timestamp (calling objectid.generation_time returns a datetime object). So it is possible to use this generation time instead of keeping a separate creation timestamp? How will you be able to sort by creation time or query for the last N items efficiently using this embedded timestamp?

推荐答案

我想因为MongoDB ObjectId包含时间戳，所以如果要按objectId进行排序，则可以按创建日期"进行排序:

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.

如果要创建最后30个项目，可以使用以下查询:

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 

我实际上不确定，我只是想按_id进行排序应如上所述.稍后我将创建一些测试.

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

更新:

是的.如果您按_id订购，则会自动按_id创建日期订购. 我已经用c#做了一个小测试，mb对它感兴趣的人:

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);
   }
}

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