gistfile1.rb

class A
  include Mongoid::Document
  embeds_one :b
end

class B
  include Mongoid::Document
  embedded_in :a
end

a = A.new(:b => B.new)
a.save
# At this point, a is persisted, with b embedded.

a.b = a.b

a.b
# => #<B _id: 504eb4cb5f28694e74000007, _type: nil>
# However, b is now deleted from the database.

a.b._id = :foo
# RuntimeError: can't modify frozen Hash

a.reload
a.b
# => nil