用于 mongodb ObjectId 创建时间
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uses for mongodb ObjectId creation time
提问by kefeizhou
The ObjectIdused 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?
该ObjectId用作MongoDB的文档的默认密钥嵌入时间戳（调用objectid.generation_time返回datetime对象）。那么可以使用这个生成时间而不是单独保留一个创建时间戳吗？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 个项目？
回答by Andrew Orsich
I suppose since MongoDB ObjectId contain a timestamp, you can sort by 'created date' if you will sort by objectId:
我想由于 MongoDB ObjectId 包含一个时间戳，如果您按 objectId 排序，则可以按“创建日期”排序：
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:
如果您想要最后 30 个创建的项目，您可以使用以下查询：
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.
我实际上不确定，我只是假设按 _id 排序应该按上述方式工作。稍后我将创建一些测试。
Update:
更新：
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:
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。我在 c# 中做了一个小测试，mb 有人对它感兴趣：
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);
}
}
回答by user105991
Yes, you can use the generation_time of BSON ObjectId for the purposes you want. So,
是的，您可以将 BSON ObjectId 的 generation_time 用于您想要的目的。所以，
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.
将返回最近创建的 10 个项目。但是，由于嵌入的时间戳具有一秒的精度，因此任何一秒内的多个项目都按其创建顺序存储。
回答by DanH
The code to convert a DateTime to its corresponding timestamp with the c# driver is as follows:
使用 c# 驱动程序将 DateTime 转换为其对应时间戳的代码如下：
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/
更多信息：http: //www.danharman.net/2011/10/26/mongodb-ninjitsu-using-objectid-as-a-timestamp/
回答by wprl
From: http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-DocumentTimestamps
来自：http: //www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-DocumentTimestamps
"sorting on an _id field that stores ObjectId values is roughly equivalent to sorting by creation time, although this relationship is not strict with ObjectId values generated on multiple systems within a single second."
“对存储 ObjectId 值的 _id 字段进行排序大致相当于按创建时间排序，尽管这种关系对于一秒钟内在多个系统上生成的 ObjectId 值并不严格。”
回答by Chen Dachao
To query projects created within 7 days, I use below snippet:
要查询在 7 天内创建的项目，我使用以下代码段：
db.getCollection('projects').find({
$where: function() {
// last 7 days
return Date.now() - this._id.getTimestamp() < (7 * 24 * 60 * 60 * 1000)
}
}).sort({
'_id': -1
})
and if you want to get items with specified fields:
如果您想获取具有指定字段的项目：
db.getCollection('projects').find({
$where: function() {
// last 7 days
return Date.now() - this._id.getTimestamp() < (7 * 24 * 60 * 60 * 1000)
}
}).sort({
'_id': -1
}).toArray().map(function(item) {
var res = {};
res['Project Name'] = item.config.label;
res['Author'] = item.author;
res['Created At'] = item._id.getTimestamp().toLocaleDateString();
res['Last Modified Date'] = item.config.lastModifDate.toLocaleString();
return res;
});
it will return something like this:
它将返回如下内容：
[{
"Project Name": "Newsletter",
"Author": "larry.chen",
"Created At": "Thursday, January 19, 2017",
"Last Modified Date": "Thursday, January 19, 2017 17:05:40"
}...]
PS:the software I use to connect to MongoDB is Robo 3T
PS：我用来连接MongoDB的软件是Robo 3T
Hope this will help you.
希望这会帮助你。
回答by Andreas Jung
See
看
http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-DocumentTimestamps
http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-DocumentTimestamps
Likely doable however I would always prefer having a dedicated timestamp instead of relying on some such internals like timestamp somehow embedded in some object id.
可能可行，但是我总是更喜欢有一个专用的时间戳，而不是依赖某些内部结构，例如以某种方式嵌入某些对象 ID 中的时间戳。
