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Daniel Doubrovkine

aka dB., CTO at artsy.net, fun at playplay.io, NYC

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Coming from mongoid-locker#57, in which I, too, wasn’t sure whether calling .utc on a timestamp was necessary with Mongoid.

The MongoDB documentation is pretty clear - MongoDB stores times in UTC by default, and will convert any local time representations into this form. The Mongoid documentation on timestamps doesn’t say anything about time zones, however its confguration docs say that you have to set use_utc to true to ensure all times are UTC in the app side.

What all of this means is that you don’t have to worry about timezones unless you’re trying to display a date/time to a user in their time zone. Just store a date/time, use it as you would use any timestamp regardless of where it came from, and call it a day.

Lets experiment a bit.

require 'mongoid'

Mongoid.connect_to 'test'

class Foo
  include Mongoid::Document
  include Mongoid::Timestamps
end

Foo.create!

The MongoDB log shows that the time is inserted in UTC, "updated_at"=>2018-08-25 10:12:58 UTC.

According to the documentation, internally, Date objects are stored as a signed 64-bit integer representing the number of milliseconds since the Unix epoch (Jan 1, 1970). The shell returns an ISO date.

$ mongo
MongoDB server version: 4.0.1
> use test
switched to db test
> db.foos.findOne()
{
  "_id" : ObjectId("5b812baa7186b5320c9346a9"),
  "updated_at" : ISODate("2018-08-25T10:12:58.514Z"),
  "created_at" : ISODate("2018-08-25T10:12:58.514Z")
}

Lets try with a well known time.

Foo.create!(created_at: Time.at(0).utc)
Foo.create!(created_at: Time.at(0))

Both are inserting 1970-01-01 00:00:00 UTC.

How about a local time?

now = Time.now
puts now

Foo.create!(created_at: now.utc)
Foo.create!(created_at: now)

My local time is 2018-08-25 12:44:37 +0200 and both insert 2018-08-25 10:44:37 UTC.

Lets query some times.

Foo.where(:created_at.gte => now)
Foo.where(:created_at.gte => now.utc)

Both are doing "created_at"=>{"$gte"=>2018-08-25 10:46:41 UTC}}.

You get the idea: it doesn’t matter whether you call .utc or not, the server will store times in UTC. This also means that you don’t know in which original timezone a timestamp was recorded from, but that the data is fully portable between timezones and that your application’s timezone can change. Finally, MongoDB has interesting non-obvious features in its aggregation framework that let you extract and group times within a specific timezone (see SO#18287493) for details.

What if you care about the timezone at save time? It looks like that would require changing how MongoDB stores the timestamp or you would have to store a timezone separately. The mongoid-metastamp gem claimed to do this, but it hasn’t been updated in a long time and doesn’t currently support any new versions of Mongoid.