Question

John Nunemaker has a blog post with some nice tips about Mongo ObjectIds — http://mongotips.com/b/a-few-objectid-tricks/ — in particular I was interested in the tip about generation_time. He suggests it’s not necessary to explicitly store the created_at time in mongo documents because you can always pull it from the ID, which caught my attention. Problem is I can’t figure out how to generate mongo queries in mongomapper to find documents based on creation time if all I have is the id.

If I store a key :created_at as part of the document I can do a query in mongomapper to get all documents created since Dec 1st like this:

Foo.where(:created_at.gt=>Time.parse("2011-12-01"))

(which maps to:

{created_at: {"$gt"=>Thu Dec 01 06:00:00 UTC 2011}}

I can’t figure out how to make the equivalent query using the ObjectId.. I imagine it’d look something like this (though obviously generation_time is a ruby function, but is there an equivalent I can use on the objectid in the context of a mongo query?)

Foo.where('$where'=>"this.id.generation_time > new Date('2011-12-01')")
{$where: "this.id.generation_time > new Date('2011-12-01')"}

One further question: if I forgo storing separate timestamps, will I lose the timestamp metadata if I dump and restore my database using mongodump? Are there recommended backup/restore techniques that preserve ObjectIds?

Answer 1

this is javascript code which would be run in the shell but generation time is a mongomapper method so it doesn’t make sense in the code you have.

In rails you would get the id by saying something like

created_at = self.id.generation_time.in_time_zone(Time.zone)

Where self refers to an instance of Foo.

And you would query by saying

Foo.find('_id' => {'$gte' => BSON::ObjectId.from_time(created_at)}).count

Why bother though… the hassle isn’t worth it, just store the time.

Regarding the backup/restore techniques, unless you are manually reading and re-inserting mongodump/restore and similar tools will preserve the object id so you have nothing to worry about there.