sample code which causes strange model interaction with multiple inheritance

example.rb

class Example
  include Mongoid::Document

  field :example_val

  class Holder < HolderBase
    embeds_one :item, :class_name => "Example"
  end
end

example_holder.rb

class ExampleHolder < HolderBase
  embeds_one :example
end

holder_base.rb

class HolderBase
  include Mongoid::Document

  field :holder_val
end

test.rb

e = Example.new(:example_val => "Example String")
 => #<Example _id: 4d3a398da435fd762a000002, example_val: "Example String"> 

e.save
 => true 

e_h = Example::Holder.new(:item => e.clone)
 => #<Example::Holder _id: 4d3a39a5a435fd762a000004, holder_val: nil> 

e_h.save

e_h.relations
 => {"example"=>#<Mongoid::Relations::Metadata
  class_name:           Example,
  cyclic:               No,
  dependent:            None,
  inverse_of:           N/A,
  inverse_setter:       =,
  inverse_type:         N/A,
  inverse_type_setter:  N/A,
  key:                  example,
  macro:                embeds_one,
  name:                 example,
  polymorphic:          No,
  relation:             Mongoid::Relations::Embedded::One,
  setter:               example=>
, "item"=>#<Mongoid::Relations::Metadata
  class_name:           Example,
  cyclic:               No,
  dependent:            None,
  inverse_of:           N/A,
  inverse_setter:       =,
  inverse_type:         N/A,
  inverse_type_setter:  N/A,
  key:                  item,
  macro:                embeds_one,
  name:                 item,
  polymorphic:          No,
  relation:             Mongoid::Relations::Embedded::One,
  setter:               item=>
} 

e_h.save
NoMethodError: undefined method `example' for #<Example::Holder _id: 4d3a39a5a435fd762a000004, holder_val: nil>
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/attributes.rb:75:in `method_missing'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:199:in `send'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:199:in `to_hash'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:198:in `each'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:198:in `to_hash'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:197:in `tap'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/document.rb:197:in `to_hash'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence/insert.rb:48:in `insert'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence/insert.rb:30:in `persist'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:413:in `_run_save_callbacks'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:93:in `send'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:93:in `run_callbacks'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence/insert.rb:29:in `persist'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:413:in `_run_create_callbacks'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:93:in `send'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/activesupport-3.0.3/lib/active_support/callbacks.rb:93:in `run_callbacks'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence/insert.rb:28:in `persist'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence.rb:45:in `insert'
	from /home/mikej/.rvm/gems/ruby-1.8.7-p302@global/gems/mongoid-2.0.0.rc.6/lib/mongoid/persistence.rb:145:in `save'
	from (irb):13

And yeah this is also another class_attribute issue I believe.

agreed, this is an odd setup, and it didn't stay that way, it was the result of artifacts to a refactoring process.

Another thing to check would be if it generates the same behavior if there is two (or more) classes which inherit from HolderBase of either a nested or non-nested type. It's possible the naming doesn't matter, but the number of things that inherit from a common base class. I don't have the time to test this today though.