Append to an array, create the array first if it doesn't already exist

announcements_controller.rb

class AnnouncementsController < ApplicationController

  # Creates an array (if needed) and stores that array in the session so we know which
  # announcements the current_user has seen
  #
  # @todo: Add test
  #
  def hide
    ( session[:announcement_ids] ||= [] ) << params[:id]
    
    render( :nothing => true, :status => :ok )
  end

end