I want to change the logic of mongoid's to_json by following code:

module Mongoid
  module Document
    def as_json(options={})
      attrs = super(options)
      attrs["id"] = self.persisted? ? self._id : nil
      attrs
    end
  end
end

where should I place it? So far I place it in application.rb and it can be loaded when starting rails. However, I want to move it to other place, where should I move to?