How do I eliminate this n+1 query? - ruby-on-rails

Question

Models:
class Device < ApplicationRecord
has_many :device_locations
has_many :locations, through: :device_locations do
def previous
order('device_locations.removed_at DESC').first
end
end
end
class Location < ApplicationRecord
has_many :device_locations
has_many :devices, through: :device_locations
end
class DeviceLocation < ApplicationRecord
belongs_to :device
belongs_to :location
end
Controller Action:
# LocationsController
def show
#location = Location.find(params[:id])
#device_locations = #location.device_locations.includes(:device).where(removed_at: nil)
end
View:
<table>
<tr><th>Name</th><th>Install Date</th><th>Uninstall Date</th><th>Previous Location</th></tr>
<%- #device_locations.each do |device_location| %>
<tr>
<td><%= link_to device_location.device.name, device_location.device %></td>
<td><%= device_location.installed_at %></td>
<td><%= device_location.removed_at %></td>
<td><%= link_to device_location.device.locations.previous.name, device_location.device.locations.previous %></td>
</tr>
<% end %>
</table>
The difficulty here is avoiding the n+1 query generated by device_location.device.locations.previous.name. It's like I need to join the table to itself?
Without this column removed from the view the log reads:
Started GET "/locations/1" for ::1 at 2019-08-24 11:09:55 -0400
(0.1ms) SELECT sqlite_version(*)
Processing by LocationsController#show as HTML
Parameters: {"id"=>"1"}
Location Load (0.4ms) SELECT "locations".* FROM "locations" WHERE "locations"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]
↳ app/controllers/locations_controller.rb:69:in `set_location'
Rendering locations/show.html.erb within layouts/application
DeviceLocation Load (14.6ms) SELECT "device_locations".* FROM "device_locations" WHERE "device_locations"."location_id" = ? AND "device_locations"."removed_at" IS NULL [["location_id", 1]]
↳ app/views/locations/show.html.erb:9
Device Load (1.0ms) SELECT "devices".* FROM "devices" WHERE "devices"."id" IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) [["id", 1], ["id", 5], ["id", 28], ["id", 35], ["id", 51], ["id", 57], ["id", 61], ["id", 86], ["id", 94], ["id", 99], ["id", 107], ["id", 115], ["id", 125], ["id", 139], ["id", 148], ["id", 151], ["id", 159], ["id", 164], ["id", 165], ["id", 168], ["id", 177]]
↳ app/views/locations/show.html.erb:9
Rendered locations/show.html.erb within layouts/application (Duration: 20.7ms | Allocations: 4172)
Completed 200 OK in 25ms (Views: 7.3ms | ActiveRecord: 15.9ms | Allocations: 6079)
With the column present it reads:
Started GET "/locations/1" for ::1 at 2019-08-24 11:10:51 -0400
Processing by LocationsController#show as HTML
Parameters: {"id"=>"1"}
Location Load (0.1ms) SELECT "locations".* FROM "locations" WHERE "locations"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]
↳ app/controllers/locations_controller.rb:69:in `set_location'
Rendering locations/show.html.erb within layouts/application
DeviceLocation Load (0.4ms) SELECT "device_locations".* FROM "device_locations" WHERE "device_locations"."location_id" = ? AND "device_locations"."removed_at" IS NULL [["location_id", 1]]
↳ app/views/locations/show.html.erb:9
Device Load (0.2ms) SELECT "devices".* FROM "devices" WHERE "devices"."id" IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?) [["id", 1], ["id", 5], ["id", 28], ["id", 35], ["id", 51], ["id", 57], ["id", 61], ["id", 86], ["id", 94], ["id", 99], ["id", 107], ["id", 115], ["id", 125], ["id", 139], ["id", 148], ["id", 151], ["id", 159], ["id", 164], ["id", 165], ["id", 168], ["id", 177]]
↳ app/views/locations/show.html.erb:9
Location Load (0.8ms) SELECT "locations".* FROM "locations" INNER JOIN "device_locations" ON "locations"."id" = "device_locations"."location_id" WHERE "device_locations"."device_id" = ? ORDER BY device_locations.removed_at DESC LIMIT ? [["device_id", 1], ["LIMIT", 1]]
↳ app/models/device.rb:5:in `previous'
CACHE Location Load (0.0ms) SELECT "locations".* FROM "locations" INNER JOIN "device_locations" ON "locations"."id" = "device_locations"."location_id" WHERE "device_locations"."device_id" = ? ORDER BY device_locations.removed_at DESC LIMIT ? [["device_id", 1], ["LIMIT", 1]]
↳ app/models/device.rb:5:in `previous'
... N times
CACHE Location Load (0.0ms) SELECT "locations".* FROM "locations" INNER JOIN "device_locations" ON "locations"."id" = "device_locations"."location_id" WHERE "device_locations"."device_id" = ? ORDER BY device_locations.removed_at DESC LIMIT ? [["device_id", 177], ["LIMIT", 1]]
↳ app/models/device.rb:5:in `previous'
Rendered locations/show.html.erb within layouts/application (Duration: 50.7ms | Allocations: 40363)
Completed 200 OK in 55ms (Views: 47.7ms | ActiveRecord: 5.3ms | Allocations: 42252)

Answer 1

Shauno, have you tried the Bullet gem? it helps you get rid of n+1 queries throughout your application by showing a notice at the bottom of pages when a n+1 query is detected, and it also suggests possible solutions.
https://github.com/flyerhzm/bullet
Just make sure to only add it to your development group in the gemfile.