gistfile1.rb

class User
  def doctors=(doctors)
    write_attribute(:cached_doctors, doctors.map(&:attributes))
    @doctors = doctors
  end
  def doctors
    @doctors ||= read_attribute(:cached_doctors).map do |doctor|
      Mongoid::Factory.build(Doctor, doctor)
    end
  end
end

gistfile2.rb

class CachedDoctors
  def self.set(doctors)
    doctors.map { |doctor| doctor.attributes }
  end
  def self.get(doctors)
    doctors.map { |doctor| Mongoid::Factory.build(Doctor, doctor) }
  end
end
User.field :doctors, :type => CachedDoctors