I had a field of the form

class Whatsit
include Mongoid::Document
references_many :hoozits, :stored_as => :array, :inverse_of => :whatsit
end

that was working fine up until recently, but now when I try to save a Whatsit instance I get

NoMethodError (undefined method `<<' for BSON::ObjectID('4c47584a321944a63100005a'):BSON::ObjectID)

Any idea what's going on? Is the syntax described in the Mongoid docs on associations no longer accurate? I'm on mongoid 2.0.0.beta9 af80db8e53d021c0543aa7940052c21b79dfde86-3a4ca6, mongo 1.0.5, mongo_ext 0.19.3, bson 1.0.4, and bson_ext 1.0.4.