???

gistfile1.rb

1.9.2p290 :007 > Mongoid::Document.send(:include, ActiveModel::SerializerSupport)
 => Mongoid::Document 
1.9.2p290 :008 > Mongoid::Document.include?(ActiveModel::SerializerSupport)
 => false 
1.9.2p290 :009 > Mongoid::Document.included_modules
 => [ProprietaryModule] 
1.9.2p290 :014 > Mongoid::Document.ancestors
 => [Mongoid::Document, ProprietaryModule] 

Mongoid is being loaded by a gem that's including the ProprietaryModule in it; this code (executed here in a rails console) should be executed inside an initializer (per http://autonomousmachine.com/posts/2012/5/5/use-active-model-serializers-with-mongoid).

Mongoid::Document doesn't appear to define its own include method, though I haven't dug through all the modules it includes.

Cannot reproduce in isolation:

fxn@cauchy:~/tmp $ cat foo.rb
require 'rubygems'
require 'mongoid'

module M
end

Mongoid::Document.send(:include, M)
p Mongoid::Document.ancestors
fxn@cauchy:~/tmp $ ruby foo.rb 2>/dev/null
[Mongoid::Document, M]

OK Alex, I didn't read your full comment, sorry :D

Could it be a 1.9.2-specific issue? What ruby version are you using, @fxn?

I'm on ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin11.3.0]. I can't rule out that it has something to do with all the other gems (including Rails) that I'm loading...but on the plus side, I've figured out how to do what I need to do in Mongoid 3 without having to switch serializers, so while I'd like to do that and figure this out, it's a lot less urgent.