Mongoid accepts_nested_attributes_for issue

I have got this relations

class Bird
  include Mongoid::Document
  has_many :eggs
  accepts_nested_attributes_for :eggs
end

class Egg
  include Mongoid::Document
  belongs_to :bird
end

bird = Bird.new
#=> 开发者_C百科#<Bird...
bird.eggs.build
#=> #<Egg...
bird.save
bird.reload.eggs
#=> []

Where are my eggs!

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Change the has_many to

has_many :eggs, :autosave => true