Hello,
It seems that sorting by datetime does not work with mongoid.

I wanted to get the last time any User logged in:

User.where(:current_sign_in_at.ne => nil).desc(:current_sign_in_at).limit(1).pluck(:current_sign_in_at)

but since the result was wrong I inquired further and:

[42] pry(main)> User.where(:current_sign_in_at.ne => nil).desc(:current_sign_in_at).entries.map(&:current_sign_in_at)
=> [Tue, 10 Mar 2015 13:05:57 CET +01:00,
Fri, 10 Apr 2015 17:35:43 CEST +02:00,
Thu, 18 Jun 2015 05:32:19 CEST +02:00,
Mon, 22 Jun 2015 13:31:29 CEST +02:00,
Wed, 13 May 2015 15:34:24 CEST +02:00,
Fri, 19 Jun 2015 12:06:37 CEST +02:00,
Fri, 19 Jun 2015 16:52:35 CEST +02:00,
Tue, 16 Jun 2015 10:18:42 CEST +02:00,
Fri, 19 Jun 2015 17:21:38 CEST +02:00,
Wed, 17 Jun 2015 21:09:05 CEST +02:00,
Wed, 27 May 2015 15:09:33 CEST +02:00,
Wed, 10 Jun 2015 10:06:51 CEST +02:00,
Wed, 27 May 2015 14:18:17 CEST +02:00,
Fri, 29 May 2015 15:46:56 CEST +02:00,
Fri, 29 May 2015 16:45:26 CEST +02:00,
Mon, 22 Jun 2015 16:14:36 CEST +02:00]

Version:

bundle show mongoid
=> /home/muichkine/.rvm/gems/ruby-2.2.2/gems/mongoid-4.0.2