foos_controller.rb

class FoosController < AbstractController::Base
  def update
    @foo = Foo.find(params[:id])
    
    # If our changed object saves...
    if @foo.update_attributes(params[:attributes])
    
      # Now, we check for changes. In some situations, we might
      # use @foo.notify_changed? but given that update_attributes
      # clobbers the dirty state of your instance, we need to
      # use previous_changes
      if @foo.previous_changes[:notify]
        # flash?
        redirect_to @foo, notice: "Flash message in flash[:notice]"
      else
        # do something else?
      end
    
    # Otherwise, handle our error
    else
      render action: "edit"
    end
  end
end