Elasticsearch / Tire - Flattening Nested Objects - ruby-on-rails

Question

In reference to the Query DSL Explained Tutorial Slides 14-15
How do I flatten Nested Objects?
I have a Model named Entry and another named Category and they share a HABTM association.
Everything is currently working and the search results seem to be correct, but I don't know if my mapping is correct. The tutorial says that when you flatten objects the Document will look like this :
{
tweet => "Perl is GREAT!",
posted => "2011-08-15",
user.name => "Clinton Gormley",
user.email => "drtech#cpan.org",
tags => ["perl","opinion"],
posts => 2,
}
with the Object user being flattened. When I look at the source of my JSON document it looks like this:
{
"title":"First",
"description":"first test",
"categories":
{"categories_name":"CAP and Using the CAP website"},
"attachment":"VEVTVCE=\n",
"published":true
}
So, I'm assuming that its supposed to say categories.categories_name but I don't know how to specify that or if that's even necessary. Here's some Model code:
class Entry < ActiveRecord::Base
include Tire::Model::Search
include Tire::Model::Callbacks
has_and_belongs_to_many :categories
mount_uploader :doc, EntryDocUploader
tire.mapping do
indexes :title
indexes :description
indexes :categories do
indexes :categories_name, type: 'string', index: 'not_analyzed'
end
indexes :attachment, :type => 'attachment',
:fields => {
:title => { :store => 'yes' },
:attachment => { :term_vector => 'with_positions_offsets', :store => 'yes' }
}
end
def to_indexed_json
{
:title => title,
:description => description,
:categories => {:categories_name => cats}, #categories.map { |c| { :categories_name => c.name}}.to_sentence,
:attachment => attachment,
}.to_json
end
def self.search(params)
tire.search(load: true) do
query { string params[:query], default_operator: "AND" } if params[:query].present?
filter :term, :published => "true"
end
end
def cats
categories.map(&:name).to_sentence
end
end