class Item
  include Mongoid::Document
  embedded_in :report
end

class Report
  include Mongoid::Document
  embeds_many :items

  after_initialize do |report|
    report.items = [Item.new, Item.new]
  end
end

NoMethodError (undefined method `find' for nil:NilClass):
  app/models/report.rb:14:in `block in <class:Report>'
  app/controllers/reports_controller.rb:6:in `new'