class User
  include Mongoid::Document
  field :name 
  belongs_to :photo
end

class Photo
  include Mongoid::Document
  field :name 
  has_one :user, dependent: :nullify
end

user = User.create(name: "john")
photo = Photo.create(name: "my cat")
user.photo = photo
user.save
photo.destroy

user = User.create(name: "john")
photo = Photo.create(name: "my cat")
user.photo = photo
user.save
photo = Photo.find("the photo's id")
photo.destroy

NameError: instance variable @photo not defined
	from mongoid-2.0.2/lib/mongoid/relations/referenced/one.rb:71:in `remove_instance_variable'
	from mongoid-2.0.2/lib/mongoid/relations/referenced/one.rb:71:in `nullify'
	from mongoid-2.0.2/lib/mongoid/relations/cascading/nullify.rb:13:in `cascade'
	from mongoid-2.0.2/lib/mongoid/relations/cascading.rb:32:in `block in cascade!'
	from mongoid-2.0.2/lib/mongoid/relations/cascading.rb:29:in `each'
	from mongoid-2.0.2/lib/mongoid/relations/cascading.rb:29:in `cascade!'
	from mongoid-2.0.2/lib/mongoid/persistence.rb:62:in `remove'
	from mongoid-2.0.2/lib/mongoid/persistence.rb:34:in `block in destroy'
	from activesupport-3.0.7/lib/active_support/callbacks.rb:419:in `_run_destroy_callbacks'
	from activesupport-3.0.7/lib/active_support/callbacks.rb:94:in `run_callbacks'
	from mongoid-2.0.2/lib/mongoid/persistence.rb:34:in `destroy'