How do I extract the created date out of a Mongo O

问题:

I'm using the Mongo shell to query my Mongo db. I want to use the timestamp contained in the ObjectID as part of my query and also as a column to extract into output. I have setup Mongo to create ObjectIDs on its own.

My problem is I can not find out how to work with the ObjectID to extract its timestamp.

Here are the queries I am trying to get working. The 'createdDate' field is a placeholder; not sure what the correct field is:

//Find everything created since 1/1/2011
db.myCollection.find({date: {$gt: new Date(2011,1,1)}});

//Find everything and return their createdDates
db.myCollection.find({},{createdDate:1});

回答1:

getTimestamp()

The function you need is this one, it's included for you already in the shell:

ObjectId.prototype.getTimestamp = function() {
    return new Date(parseInt(this.toString().slice(0,8), 16)*1000);
}

References

Check out this section from the docs:

• Extract insertion times from _id rather than having a separate timestamp field

This unit test also demostrates the same:

• mongo / jstests / objid6.js

Example using the Mongo shell:

> db.col.insert( { name: "Foo" } );
> var doc = db.col.findOne( { name: "Foo" } );
> var timestamp = doc._id.getTimestamp();

> print(timestamp);
Wed Sep 07 2011 18:37:37 GMT+1000 (AUS Eastern Standard Time)

> printjson(timestamp);
ISODate("2011-09-07T08:37:37Z")

回答2:

This question is helpful to understand of how to use the _id's embedded timestamp in query situations (refers to the Mongo Extended JSON documentation). This is how it's done:

col.find({..., 
     '_id' : {'$lt' : {'$oid' : '50314b8e9bcf000000000000'}} 
})

finds documents created earlier than the one that's given by oid. Used together with natural sorting and limiting you can utilize BSON _ids to create Twitter-like API queries (give me the last OID you have and I'll provide twenty more)

回答3:

In python you can do this:

>>> from bson.objectid import ObjectId
>>> gen_time = datetime.datetime(2010, 1, 1)
>>> dummy_id = ObjectId.from_datetime(gen_time)
>>> result = collection.find({"_id": {"$lt": dummy_id}})

I think, ObjectId.from_datetime() - its a useful method of standard bson lib Maybe other language bindings have alternative builtin function. Source: http://api.mongodb.org/python/current/api/bson/objectid.html