When was a document added to a MongoDB collection

Question

I have an existing mongodb collection, which doesn't have any information about when the document was created.

Is it possible to get this information some how? I've had a look through the docs but can't see it anywhere.

Answer 1

If you are using the default ObjectId value for your _id attribute, the creation time is encoded inside it.

As stated in the ObjectID documentation:

ObjectId is a 12-byte BSON type, constructed using:

• a 4-byte value representing the seconds since the Unix epoch,
• a 3-byte machine identifier
• a 2-byte process id, and a 3-byte counter, starting with a random value.

You can call the getTimestamp() function on an ObjectId object to get an ISODate object containing the creation time of the object:

In the mongo shell:

ObjectId().getTimestamp()
ISODate("2014-05-14T14:29:12Z")

Answer 2

There is most likely the actual _id value of the document unless you have replaced this with something else.

In the simple "JavaScript" syntax of this ( and various methods are available to other languages) you simply access this as:

var id = new ObjectId();
id.getTimetstamp();

Various language implementations have a way of retrieving the "timestamp" from an ObjectId value so you can just use that.