copy_script.rb

#!ruby
# w = Station.first.build_worker(:reward => 2.0, :number => 1)
# w.save!

# Each station worker is copied to human_worker which is inherited from dummy_worker (later is referenced in station)

Station.all.each do |station|
  worker = station.worker
  human_worker = HumanWorker.new(:number => worker.number, :reward => worker.reward, :worker_type => worker.worker_type, :station_id => station.id)
  station.dummy_worker = human_worker
  human_worker.save!
  station.worker.delete
  station.save!
end

dummy_worker.rb

# same as worker.rb except class name
class DummyWorker
  include Mongoid::Document

  # Fields
  field :worker_type
  field :reward, :type => Float
  field :number, :type => Integer, :default => 1

  # Associations
  referenced_in :station

  # validates :reward, :human_reward => true

end

human_worker.rb

# human_woker inherits from dummy_worker which is referenced_in station
class HumanWorker < DummyWorker
  include Mongoid::Document
  
  validates :reward, :format => {:with  => /^[0-9]*\.?[0-9]{1,2}$/, :message  => "must be numeric with two digit at most after decimal"} 
end

station.rb

# added referenced dummy_worker
class Station
  include Mongoid::Document
  include Mongoid::Timestamps

  # Fields
  field :station_type

  # Associations
  embeds_one :instruction

  # RobotWorker which inherits from worker is referenced_in Robot so we need reference worker instead of embedding it.
  embeds_one :worker

  # add referenced dummy_worker to copy embedded worker
  references_one :dummy_worker

  references_many :station_instances
  referenced_in :line
  
  # Validations
  validates :station_type, :presence => true, :on => :create

end