class Foo
  include Mongoid::Document
  has_many :bars
end
class Bar
  include Mongoid::Document
  belongs_to :foo
end

f = Foo.create!
 => #<Foo _id: 4ef07b860bed3f9672000001, _type: nil> 
Bar.create! :foo => f
Mongo::OperationFailure: 11000: E11000 duplicate key error index: blah_development.bars.$_id_  dup key: { : ObjectId('4ef07b8a0bed3f9672000002') }