mp.rb

class MagicalPresenter < Draper::Base
  extend ActiveModel::Naming
  include ActiveModel::Conversion

  attr_reader :changes
  def initialize(model, opts = {})
    @changes = {}
    super(model)
  end

  delegate :id, to: :model
  delegate :persisted?, to: :model

  def child
    model.foobar || Foobar.new(:parent => foobar) # GOTCHA: make sure and set the inverse on the relation, so the parent gets its foobar set
  end

  def child_location=(attrib)
    child.location = attrib
  end

  def child_location
    child.location # might be able to lean on delegate here
  end

  def update_attributes(attributes)
    attributes.each { |key, value|
      self.send("#{key}=", value.presence) # because draper, these will get passed through to the parent model
    }
  end

end

mp_controller.rb

def update
  @model = MagicalPresenter.new(parent.find(params[:id]))
  @model.update_attributes(params[:model])
  @model.save
end