[3.1.0.rc4] Problem with reflections being empty causing whiny nil

Hi, I have an engine that worked perfectly on rails 3.0.x and now is
breaking on a simple method.
Basically it creates categories that can have subcategories but all
with the same model.

class Category < ActiveRecord::Base
  belongs_to :parent, :class_name => 'Category'
  has_many :subcategories, :class_name => 'Category', :foreign_key
=> :parent_id

basically, what is happening is, everytime I call the
category.subcategories method
I get this error:

NoMethodError: undefined method `association_class' for nil:NilClass
  from /Users/draiken/.rvm/gems/ruby-1.9.2-p180@eujafui/gems/
  from /Users/draiken/.rvm/gems/ruby-1.9.2-p180@eujafui/gems/
  from /Users/draiken/.rvm/gems/ruby-1.9.2-p180@eujafui/gems/
association.rb:41:in `block in define_readers'

I've noticed for some reason, category.reflections is empty

I've tried debugging and I really don't know what is wrong, since
nothing changed besides the rails 3.1

Thanks in advance

Discovered it was an incompatibility issue with another gem ‘has_many_polymorphs’