How do you set _parent using Tire in a Rails model?

Apologies for my dense-ity. Elasticsearch & Tire have my head spinning...

I'm looking for an example of how to index a parent relationship between a
Page and a Report model in Rails.

Should I use the report_id attribute of Page and index it as "_parent" or
do something like Karmi suggests here
And if so will the _parent relationship be established automatically?

In my Page model I have:

class Page < ActiveRecord::Base
include Tire::Model::Search
include Tire::Model::Callbacks

belongs_to :report
attr_accessible :filename, :page_num, :parent_code

tire.mapping do
indexes :id, :type => 'integer'
indexes :report_id, :type => 'integer' # but how is this
associated with _parent?
indexes :parent_code