If you're using respond_to in your application to expose records in xml...

class WidgetsController

  def index
    @widgets = current_account.widgets
    respond_to do |format|
      format.html
      format.xml { render :xml => @widgets }
    end
  end

  def show
    @widget = current_account.widgets.find params[:id]
    respond_to do |format|
      format.html
      format.xml { render :xml => @widget }
    end
  end

  ...

end

... and are wishing to eliminate meaningless (for the user) columns in the output (like account_id)...

<?xml version="1.0" encoding="UTF-8"?>
<widget>
  <account-id type="integer">123</account-id>
  <name>foo</name>
</widget>

you can easily drop the following in config/initializers:

active_record_ext.rb

class ActiveRecord::Base
  def to_xml_with_filtering(options={})
    defaults = { :except => :account_id }
    to_xml_without_filtering defaults.merge(options)
  end
  alias_method_chain :to_xml, :filtering
end

and all your objects will filter the default columns.