class A
  include Mongoid::Document
  belongs_to :b
  belongs_to :pol, :polymorphic => true
end

class B
  include Mongoid::Document
  has_many :a
end

class C
  include Mongoid::Document
  has_one :a, :as => :pol   # BTW, problem disappear with has_many
end

A.create(:b => B.create, :pol => C.create)
# The error occurred while evaluating nil.push
# … /.rvm/gems/ruby-1.9.2-head/gems/mongoid-2.1.2/lib/mongoid/relations/bindings/referenced/in.rb:32:in `block in bind'

A.create(:pol => C.create, :b => B.create)
# OK