class ApiKey
  include Mongoid::Document
  
  field :access_token, type: String 
  embedded_in :user
  before_create :generate_access_token
  
  private
    def generate_access_token
      self.access_token = SecureRandom.hex
    end
end

class User 
  include Mongoid::Document

  field :name, type: String
  embeds_one :apikey
end

u = User.new name: 'foo'
a = ApiKey.new

puts u.inspect 
#<User _id: 54ecd47b6c6f63241d000000, name: "foo">

puts a.inspect
#<ApiKey _id: 54ecdade6c6f6324fc010000, access_token: nil>

u.apikey = a   # Here raise the error!