http://vakhov.me/blog/2015/01/26/flat_map/index.html#comment-1819549117

test.rb

require "rubygems"
require "active_record"

ActiveRecord::Base.establish_connection "postgres://localhost/geo"

ActiveRecord::Base.connection.create_table("planets") do |t|
  t.string :name
end

ActiveRecord::Base.connection.create_table("countries") do |t|
  t.belongs_to :planet
  t.string :name
end

ActiveRecord::Base.connection.create_table("cities") do |t|
  t.belongs_to :country
  t.string :name
end

class Planet < ActiveRecord::Base
  has_many :countries
end

class Country < ActiveRecord::Base
  has_many :cities
end

class City < ActiveRecord::Base
end

Planet.create!(name: "Earth").tap { |planet|
  planet.countries.create!(name: "Russia").tap { |ru|
    ru.cities.create!(name: "Moscow")
    ru.cities.create!(name: "Perm")
  }
}

Planet.create!(name: "Mars").tap { |planet|
  planet.countries.create!(name: "Red Country").tap { |ru|
    ru.cities.create!(name: "Red City")
  }
}

p Planet.
  includes(:countries => :cities).
  flat_map(&:countries).flat_map(&:cities).
  map(&:name)

# => ["Moscow", "Perm", "Red City"]