Use your MongoDB `_id` field as your created at timestamp and you can get away without creating an index for the created_at field

From time to time I have to run queries against our MongoDB collections picking documents created over specific time ranges but not all of those collections have an index at the created_at field. This led to some of them taking too long to run even on our secondaries or the hidden replica.

This changed when I was presented to the fact that the ObjectId values produced by Mongo already include a timestamp on them!

I could just use the collection _id field to do date range queries and since the field is indexed by default I didn’t even have to care about indexing it myself.

If you’re using Mongoid, a simple way of creating these values is using the Moped::BSON::ObjectId.from_time method. Let’s look at an example that finds all items created today:

today_object_id = Moped::BSON::ObjectId.from_time(Time.now.utc.at_beginning_of_day) # 5407ab800000000000000000 for "2014-09-04 00:00:00 UTC"
items_created_today = MongoidModel.where(:_id.gte => today_object_id)

This finds all items that were created after the date you provided (at_beginning_of_day is a method that’s included by ActiveSupport at Time objects) and it should be really quick due to the index at _id that’s already there all the time.

If you have to go through a lot of data and don’t want to keep cursors alive at the server (avoid long running cursors on MongoDB) you can use this same method to build a manual cursor that loads all items for every day for 30 days and does stuff to them. Here’s how it would look like:

last_30_days = 29.times.inject([30.days.ago.utc.at_beginning_of_day]) do |acc,_|
  acc << acc.last.advance(days: 1)
  acc
end

last_30_days.each do |day|
  start_object_id = Moped::BSON::ObjectId.from_time(day)
  end_object_id = Moped::BSON::ObjectId.from_time(day.advance(days: 1))  
  items = MongoidModel.where(:_id.gte => start_object_id).where(:_id.lt => end_object_id)
  # do something with the items here
end

Here we find any items that were created at that day until the beginning of the next one. You can build any kind of date range query against ObjectId fields just by making use of the from_time method, if you can create a Time object, you can produce an ObjectId value to use it as a filter.

This also works if you’re using the main MongoDB driver, just use the BSON::ObjectId.from_time method that works exactly the same way. Give it a Time object and it will give you the ObjectId value that represents that timestamp.

And if you’d like to get an ObjectId without writing any Ruby code at all, use Steve Ridout’s ObjectId generator and you should be good to go.

Now stop indexing your created_at field :)

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