Optimize eager loading in Rails - ruby-on-rails

Question

Rails 3.2. I have the following:
# city.rb
class City < ActiveRecord::Base
has_many :zones, :dependent => :destroy
end
# zone.rb
class Zone < ActiveRecord::Base
belongs_to :city
has_many :zone_shops, :dependent => :destroy
has_many :shops, :through => :zone_shops
end
# zone_shop.rb
class ZoneShop < ActiveRecord::Base
belongs_to :zone
belongs_to :shop
end
# shop.rb
class Shop < ActiveRecord::Base
end
# cities_controller.rb
def show
#trip = City.find(params[:id], :include => [:user, :zones => [:shops]])
#zones = #trip.zones.order("position")
# List out all shops for the trip
shops_list_array = []
#zones.each do |zone, i|
zone.shops.each do |shop|
shops_list_array << shop.name
end
end
#shops_list = shops_list_array.join(', ')
end
# development.log
City Load (0.3ms) SELECT `cities`.* FROM `cities` WHERE `cities`.`id` = 1 LIMIT 1
Zone Load (0.3ms) SELECT `zones`.* FROM `zones` WHERE `zones`.`trip_id` IN (1) ORDER BY position asc
ZoneShop Load (0.3ms) SELECT `zone_shops`.* FROM `zone_shops` WHERE `zone_shops`.`zone_id` IN (26, 23, 22) ORDER BY position asc
Shop Load (0.5ms) SELECT `shops`.* FROM `shops` WHERE `shops`.`id` IN (8, 7, 1, 9)
Zone Load (0.5ms) SELECT `zones`.* FROM `zones` WHERE `zones`.`trip_id` = 1 ORDER BY position asc, position
Shop Load (0.5ms) SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 26
Shop Load (0.6ms) SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 23
Shop Load (0.4ms) SELECT `shops`.* FROM `shops` INNER JOIN `zone_shops` ON `shops`.`id` = `zone_shops`.`spot_id` WHERE `zone_shops`.`zone_id` = 22
Notice in my log, the last 3 lines with shops with ID 26, 23, 22 are redundant. How should I rewrite my cities_controller.rb to reduce the query to the system?
Many thanks.

Answer 1

#zones = #trip.zones.includes(:shops).order("position")
This eager-loads the shops association and should elimitate the n+1 query problem caused by zone.shops.each
For more information, have a look at the Ruby on Rails Guide section 12 on Eager Loading associations, which was also linked by #Benjamin M

Answer 2

I'd suggest that this
zone.shops.each do |shop|
shops_list_array << shop.name
end
produces the 3 last lines of your log. This means: You currently have only one zone inside your database. If you put more zones in there, you will get a lot more Zone Load entries in log.
The problem obviously is Rails' each method, which triggers the lazy loading:
#zones.each do |zone, i|
...
The solution depends on your needs, but I'd suggest that you read everything about Rails' eager loading feature. (There's exactly your problem: The each thing). Look it up here: http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations
It's pretty easy, short and straightforward :)