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

One further note.. if the inheritance on HolderBase is removed, things work just fine as well. My first attempt to reproduce the issue didn't reproduce the bug for that very reason. So it's the creative inheritance involved which trips things up.

Thanks for the gist... This is an odd way of modeling things to me, but the first thing I can see is Example needs an embedded_in defined... But I'll need to mess around with this a bit to see what exactly needs to be done.

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.