class LastActivity < ActiveRecord::Migration
  def self.up
		add_column :elts, :last_activity, :datetime
		execute 'UPDATE elts SET last_activity = (SELECT MAX(created_on) FROM elts c WHERE elts.lft <= c.lft AND c.rgt <= elts.rgt)'
  end

  def self.down
		remove_column :elts, :last_activity
  end
end