mongoid_question.rb

class Person
  include Mongoid::Document
end
class Preference
  include Mongoid::Document
end

shawn = Person.create
 => #<Person _id: 4d6d10867654aa7574000001, > 

# add relationship
class Person
  include Mongoid::Document
  references_and_referenced_in_many :preferences
end
class Preference
  include Mongoid::Document
  references_and_referenced_in_many :people
end

# works fine with a new Person
Person.create.preferences << Preference.create

# a pre-existing persion blows up
Person.find("4d6d10867654aa7574000001").preferences << Preference.create

Person.first.preferences << Preference.create 

# error from rc.5
TypeError: can't dup NilClass
	from org/jruby/RubyKernel.java:1874:in `dup'
	from /Users/shawn42/.rvm/gems/jruby-1.6.0.RC2/gems/mongoid-2.0.0.rc.5/lib/mongoid/relations/builders/referenced/many_to_many.rb:18:in `build'
	from /Users/shawn42/.rvm/gems/jruby-1.6.0.RC2/gems/mongoid-2.0.0.rc.5/lib/mongoid/relations/accessors.rb:59:in `create_relation'
	from /Users/shawn42/.rvm/gems/jruby-1.6.0.RC2/gems/mongoid-2.0.0.rc.5/lib/mongoid/relations/accessors.rb:26:in `build'
	from /Users/shawn42/.rvm/gems/jruby-1.6.0.RC2/gems/mongoid-2.0.0.rc.5/lib/mongoid/relations/accessors.rb:116:in `preferences'
	from org/jruby/RubyProc.java:268:in `call'

# error from rc.7
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.<<