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

问题描述

在 mongodb 文档中用作默认键的 ObjectId 嵌入了时间戳(调用 objectid.generation_time 返回一个 datetime 对象).那么是否可以使用这个生成时间而不是单独保存一个创建时间戳呢?您将如何使用此嵌入式时间戳有效地按创建时间排序或查询最后 N 项?

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

我想由于 MongoDB ObjectId 包含时间戳，如果您按 objectId 排序，则可以按创建日期"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.

如果您想要最后 30 个创建的项目，您可以使用以下查询:

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，我只是假设按 _id 排序应该如上所述.我稍后会创建一些测试.

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，确实如此.如果您按 _id 订购，您将自动按 _id 创建日期订购.我用 c# 做过小测试，有人对它感兴趣:

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);
   }
}

这篇关于用于 mongodb ObjectId 创建时间的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！