It happens on Mongoid 3.1.6 with Rails 3.2.16. Here are the steps to reproduce:

1. we have some inheritance models with accepts_nested_attributes_for option (see the full example: https://github.com/zuzara/test-rails-mongoid-inheritance/tree/master/app/models)
2. run rails server (development mode)
3. create Product through your browser (access /products)
4. there should be no error
5. edit any model file
6. create Product again
7. get Mongoid::Errors::UnknownAttribute

It happens on Product.new({detail_attributes: {_type: "Laptop", price: 10)}}
Laptop.superclass == Detail will be false when it gets reloaded. I see superclass mismatch for class Laptop error, then Mongoid::Errors::UnknownAttribute is threw.

Mongoid seems modifying superclass in lib/mongoid/hierarchy.rb, maybe that's why the model can't be overwritten but I couldn't figure out the exact reason. The problem will be gone if I add require_dependency for subclasses in the parent class. (in detail.rb in the example)

Am I missing something or is the behavior just as designed? I've found this post on Rails side, so wasn't sure if it's a bug - https://github.com/rails/rails/issues/8699