Use size instead of count for has many field

Created by: BenMorganIO

To help improve the admin performance, I noticed that there was a long string of #count queries going on; see before for the issue. The odd part to me was that I was doing this in my code: Course.includes(:videos) and yet there was a constant call to #count going on. Since the AR association is already loaded, why go back to PG? Switched from using #count to #size so that AR can intelligently load up the data and call count if its not there or, if it is there, just use ruby.

Before:

17:50:43 web.1    | Started GET "/admin/courses" for ::1 at 2015-11-21 17:50:43 -0800
17:50:43 web.1    | Processing by Admin::CoursesController#index as HTML
17:50:43 web.1    |   Course Load (0.4ms)  SELECT  "courses".* FROM "courses" LIMIT 20 OFFSET 0
17:50:43 web.1    |   Video Load (0.4ms)  SELECT "videos".* FROM "videos" WHERE "videos"."course_id" IN (13, 14, 15, 16)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.1ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.1ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:50:43 web.1    |    (0.2ms)  SELECT COUNT(*) FROM "videos" WHERE "videos"."course_id" = $1  [["course_id", 13]]
17:50:43 web.1    |   Rendered fields/has_many/_index.html.erb (0.8ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:50:43 web.1    |    (0.2ms)  SELECT COUNT(*) FROM "videos" WHERE "videos"."course_id" = $1  [["course_id", 14]]
17:50:43 web.1    |   Rendered fields/has_many/_index.html.erb (1.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:50:43 web.1    |    (0.1ms)  SELECT COUNT(*) FROM "videos" WHERE "videos"."course_id" = $1  [["course_id", 15]]
17:50:43 web.1    |   Rendered fields/has_many/_index.html.erb (0.8ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.0ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.1ms)
17:50:43 web.1    |    (0.1ms)  SELECT COUNT(*) FROM "videos" WHERE "videos"."course_id" = $1  [["course_id", 16]]
17:50:43 web.1    |   Rendered fields/has_many/_index.html.erb (0.9ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_collection.html.erb (45.2ms)
17:50:43 web.1    |    (0.3ms)  SELECT COUNT(*) FROM "courses"
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/index.html.erb within layouts/administrate/application (59.9ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_sidebar.html.erb (0.7ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_flashes.html.erb (0.1ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_javascript.html.erb (53.1ms)
17:50:43 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/layouts/administrate/application.html.erb (187.3ms)
17:50:43 web.1    | Completed 200 OK in 205ms (Views: 202.2ms | ActiveRecord: 1.7ms)

After:

17:51:21 web.1    | Started GET "/admin/courses" for ::1 at 2015-11-21 17:51:21 -0800
17:51:21 web.1    | Processing by Admin::CoursesController#index as HTML
17:51:21 web.1    |   Course Load (0.4ms)  SELECT  "courses".* FROM "courses" LIMIT 20 OFFSET 0
17:51:21 web.1    |   Video Load (0.4ms)  SELECT "videos".* FROM "videos" WHERE "videos"."course_id" IN (13, 14, 15, 16)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered fields/has_many/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered fields/has_many/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered fields/has_many/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/number/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/string/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/fields/text/_index.html.erb (0.0ms)
17:51:21 web.1    |   Rendered fields/has_many/_index.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_collection.html.erb (41.2ms)
17:51:21 web.1    |    (0.2ms)  SELECT COUNT(*) FROM "courses"
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/index.html.erb within layouts/administrate/application (55.5ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_sidebar.html.erb (0.8ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_flashes.html.erb (0.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/administrate/application/_javascript.html.erb (51.1ms)
17:51:21 web.1    |   Rendered /Users/BenAMorgan/.rvm/gems/ruby-2.2.3/bundler/gems/administrate-a14e388bed54/app/views/layouts/administrate/application.html.erb (182.7ms)
17:51:21 web.1    | Completed 200 OK in 201ms (Views: 198.6ms | ActiveRecord: 1.0ms)

Reference: http://batsov.com/articles/2014/02/17/the-elements-of-style-in-ruby-number-13-length-vs-size-vs-count/