Object-Data getting lost

When iterating with a cursor over a set of Mongoid Documents, the changes to those objects are lost outside the loop.
Is this the expected behaviour?

Please have a look at 'spec/computer_spec.rb' in https://github.com/QEDio/mongoid_error

Run the spec with: rake

What?

class Computer
include Mongoid::Document
field :active, type: Boolean
field :dirty, type: Boolean
end

This fails:

computers = Computer.all

1. computers = Computer.all.to_a fixes the problem

computers.each do |c|
c.active = true
c.dirty = false
end

computers.each do |c|
c.save
end

1. nothing gets updated