atomic increment results in dupes

[ben....@gmail.com]

I’m using Mongoid 4.0.0.alpha2 in a Rails 4 app.  I have a Project model that looks something like this:

class Project
  include Mongoid::Document
  field :last_serial_number, type: Integer, default: 0

  def next_serial_number
    inc(last_serial_number: 1). last_serial_number
  end
end

From what I understand of inc(), calling next_serial_number repeatedly should result in a unique, ever-incrementing int.  Usually it works great.  But sometimes when the app is under heavy load the same serial is returned twice, resulting in dupes.

The app is running on a mongo cluster that include three servers.  At first I thought the dupes might have something to do with master/replication skew, so I added 

 

Mongoid::Config.sessions['default'] ||= {}
Mongoid::Config.sessions['default']['options'] ||= {}
Mongoid::Config.sessions['default']['options']['read'] = 'primary'

 

to an initializer.  This didn’t help at all.  The app is also uses Sidekiq to process background jobs, and I thought that maybe there was some kind of threading problem with atomic increments.  The Kiqstand readme, though, seems to suggest that Mongoid 4 should correctly pool connections for Sidekiq out of the box.

Is there something obvious I’m doing wrong here?  Could this be some kind of Mongoid 4 bug?