ruby
require 'mongoid'

Mongoid.configure {|c| c.connect_to "test"}

puts "Using Mongoid #{Mongoid::VERSION}"

class Assignment
  include Mongoid::Document

  field :external_1, type: Integer, default: 0
  field :external_2, type: Integer, default: 0
  field :internal_1, type: Array
  field :internal_2, type: Mongoid::Boolean, default: true

  def my_push value
    atomically do
      add_to_set internal_1: value
      unset :internal_2
    end
  end
end
Assignment.delete_all

Moped.logger = Logger.new(STDOUT)

assignment = Assignment.create
assignment.atomically do |a|
  a.inc external_1: 2
  a.inc external_2: 4
  a.my_push "qwe"
end

mongoid-errors$ruby nested_atomically.rb 
Using Mongoid 4.0.1
D, [2015-01-30T01:35:02.833221 #7624] DEBUG -- :   MOPED: 127.0.0.1:27017 INSERT       database=test collection=assignments documents=[{"_id"=>BSON::ObjectId('54cad1b64164611dc8000000'), "external_1"=>0, "external_2"=>0, "internal_2"=>true}] flags=[]
D, [2015-01-30T01:35:02.833285 #7624] DEBUG -- :                          COMMAND      database=test command={:getlasterror=>1, :w=>1} runtime: 0.2760ms
D, [2015-01-30T01:35:02.833866 #7624] DEBUG -- :   MOPED: 127.0.0.1:27017 UPDATE       database=test collection=assignments selector={"_id"=>BSON::ObjectId('54cad1b64164611dc8000000')} update={"$addToSet"=>{"internal_1"=>{"$each"=>["qwe"]}}, "$unset"=>{"internal_2"=>true}} flags=[]
D, [2015-01-30T01:35:02.833898 #7624] DEBUG -- :                          COMMAND      database=test command={:getlasterror=>1, :w=>1} runtime: 0.2960ms
D, [2015-01-30T01:35:02.835907 #7624] DEBUG -- :   MOPED: 127.0.0.1:27017 UPDATE       database=test collection=assignments selector={"_id"=>BSON::ObjectId('54cad1b64164611dc8000000')} update=nil flags=[]
D, [2015-01-30T01:35:02.835944 #7624] DEBUG -- :                          COMMAND      database=test command={:getlasterror=>1, :w=>1} runtime: 1.8620ms
/usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/operation/read.rb:50:in `block in execute': The operation: #<Moped::Protocol::Command (Moped::Errors::OperationFailure)
  @length=68
  @request_id=10
  @response_to=0
  @op_code=2004
  @flags=[]
  @full_collection_name="test.$cmd"
  @skip=0
  @limit=-1
  @selector={:getlasterror=>1, :w=>1}
  @fields=nil>
failed with error "d.moreJSObjs()"
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:594:in `[]'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:594:in `block (2 levels) in flush'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:593:in `map'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:593:in `block in flush'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:617:in `block in logging'
	from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/notifications.rb:164:in `block in instrument'
	from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
	from /usr/local/lib/ruby/gems/2.2.0/gems/activesupport-4.2.0/lib/active_support/notifications.rb:164:in `instrument'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/instrumentable.rb:31:in `instrument'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:616:in `logging'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:587:in `flush'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:358:in `pipeline'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/operation/write.rb:47:in `execute'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:665:in `write'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:502:in `update'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/query.rb:427:in `block in update'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/cluster.rb:249:in `block in with_primary'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:204:in `block in ensure_primary'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/executable.rb:25:in `execute'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/node.rb:203:in `ensure_primary'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/cluster.rb:248:in `with_primary'
	from /usr/local/lib/ruby/gems/2.2.0/gems/moped-2.0.3/lib/moped/query.rb:426:in `update'
	from /usr/local/lib/ruby/gems/2.2.0/gems/mongoid-4.0.1/lib/mongoid/query_cache.rb:117:in `update_with_clear_cache'
	from /usr/local/lib/ruby/gems/2.2.0/gems/mongoid-4.0.1/lib/mongoid/persistable.rb:212:in `persist_atomic_operations'
	from /usr/local/lib/ruby/gems/2.2.0/gems/mongoid-4.0.1/lib/mongoid/persistable.rb:60:in `atomically'
	from nested_atomically.rb:27:in `<main>'