rails activerecord relation join tables - ruby-on-rails

Question

i have a country model and a travel note model. A country has many travel notes and a travel note belongs to one country.
class TravelNote < ActiveRecord::Base
default_scope { order(created_at: :desc) }
belongs_to :country
has_many :chapters
before_destroy { draft? }
validate :validates_uniqueness_of_draft, on: :create
enum status: { draft: 0, published: 1, archived: 2 }
enum advice_against: { no: 0, general: 1, tourists: 2 }
scope :country, ->(country_id) { where(country_id: country_id) }
# further methods omitted...
end
class Country < ActiveRecord::Base
default_scope { order(:iso_code) }
has_many :travel_notes
end
in app/controllers/countries_controller.rb:
class CountriesController < ApplicationController
def index
#countries = Country.includes(:travel_notes)
end
# rest of the class omitted...
end
in app/views/countries/index.html.haml:
#countries.each do |country|
%tr
%td= link_to country.name_de, country_travel_notes_path(country)
%td= TravelNote.published.country(country.id).first.try :published_at
because of performance reason i want to remove TravelNote.published.country(country.id).first.try :published_at so that there is not hundreds of database queries anymore instead just an array of an equivalent sql query:
select * from countries
left join travel_notes
on countries.id=travel_notes.country_id
how can i achieve it?

Answer 1

Apparently you are trying to eager-load the "travel_notes" associated to the country:
Country.includes(:travel_notes).where(travel_notes: { status: 1} )
so your code will be:
class CountriesController < ApplicationController
def index
#countries = Country.includes(:travel_notes).where(travel_notes: { status: 1} )
end
# rest of the class omitted...
end
#countries.each do |country|
%tr
%td= link_to country.name_de, country_travel_notes_path(country)
%td= country.travel_notes.first.published_at

Answer 2

You could write a custom scope to include only the published notes.
something like
scope :include_published, -> { proc do |_|
joins("LEFT OUTER JOIN (
SELECT b.*
FROM travel_notes b
WHERE published = 1
GROUP BY b.country_id
) notes_select ON notes_select.country_id = countries.id"
).select("countries.*").select("#{insert what attributes you want to include }")
end.call(self, counted_model) }
You have to include the attributes you want from the note in the second select clause then they will be included in the country active record result as methods with the same name.
The SQL-query can be written prettier, I just threw something together...
I use a similar technique in my project but in order to include counts of associated objects.